Put functional tests in `tests/functional`

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

(cherry picked from commit 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 146 insertions, 0 deletions

diff --git a/tests/functional/lang.sh b/tests/functional/lang.sh
new file mode 100755
index 000000000..f4760eced
--- /dev/null
+++ b/tests/functional/lang.sh
@@ -0,0 +1,146 @@
+source common.sh
+
+set -o pipefail
+
+source lang/framework.sh
+
+# specialize function a bit
+function diffAndAccept() {
+    local -r testName="$1"
+    local -r got="lang/$testName.$2"
+    local -r expected="lang/$testName.$3"
+    diffAndAcceptInner "$testName" "$got" "$expected"
+}
+
+export TEST_VAR=foo # for eval-okay-getenv.nix
+export NIX_REMOTE=dummy://
+export NIX_STORE_DIR=/nix/store
+
+nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>&1 | grepQuiet Hello
+nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>/dev/null | grepQuiet 123
+nix-instantiate --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1
+nix-instantiate --trace-verbose --eval -E 'builtins.traceVerbose "Hello" 123' 2>&1 | grepQuiet Hello
+nix-instantiate --eval -E 'builtins.traceVerbose "Hello" 123' 2>&1 | grepQuietInverse Hello
+nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" 123' 2>&1 | grepQuietInverse Hello
+expectStderr 1 nix-instantiate --show-trace --eval -E 'builtins.addErrorContext "Hello" (throw "Foo")' | grepQuiet Hello
+
+nix-instantiate --eval -E 'let x = builtins.trace { x = x; } true; in x' \
+  2>&1 | grepQuiet -E 'trace: { x = «potential infinite recursion»; }'
+
+nix-instantiate --eval -E 'let x = { repeating = x; tracing = builtins.trace x true; }; in x.tracing'\
+  2>&1 | grepQuiet -F 'trace: { repeating = «repeated»; tracing = «potential infinite recursion»; }'
+
+set +x
+
+badDiff=0
+badExitCode=0
+
+for i in lang/parse-fail-*.nix; do
+    echo "parsing $i (should fail)";
+    i=$(basename "$i" .nix)
+    if expectStderr 1 nix-instantiate --parse - < "lang/$i.nix" > "lang/$i.err"
+    then
+        diffAndAccept "$i" err err.exp
+    else
+        echo "FAIL: $i shouldn't parse"
+        badExitCode=1
+    fi
+done
+
+for i in lang/parse-okay-*.nix; do
+    echo "parsing $i (should succeed)";
+    i=$(basename "$i" .nix)
+    if
+        expect 0 nix-instantiate --parse - < "lang/$i.nix" \
+            1> "lang/$i.out" \
+            2> "lang/$i.err"
+    then
+        sed "s!$(pwd)!/pwd!g" "lang/$i.out" "lang/$i.err"
+        diffAndAccept "$i" out exp
+        diffAndAccept "$i" err err.exp
+    else
+        echo "FAIL: $i should parse"
+        badExitCode=1
+    fi
+done
+
+for i in lang/eval-fail-*.nix; do
+    echo "evaluating $i (should fail)";
+    i=$(basename "$i" .nix)
+    if
+        expectStderr 1 nix-instantiate --show-trace "lang/$i.nix" \
+            | sed "s!$(pwd)!/pwd!g" > "lang/$i.err"
+    then
+        diffAndAccept "$i" err err.exp
+    else
+        echo "FAIL: $i shouldn't evaluate"
+        badExitCode=1
+    fi
+done
+
+for i in lang/eval-okay-*.nix; do
+    echo "evaluating $i (should succeed)";
+    i=$(basename "$i" .nix)
+
+    if test -e "lang/$i.exp.xml"; then
+        if expect 0 nix-instantiate --eval --xml --no-location --strict \
+                "lang/$i.nix" > "lang/$i.out.xml"
+        then
+            diffAndAccept "$i" out.xml exp.xml
+        else
+            echo "FAIL: $i should evaluate"
+            badExitCode=1
+        fi
+    elif test ! -e "lang/$i.exp-disabled"; then
+        declare -a flags=()
+        if test -e "lang/$i.flags"; then
+            read -r -a flags < "lang/$i.flags"
+        fi
+
+        if
+            expect 0 env \
+                NIX_PATH=lang/dir3:lang/dir4 \
+                HOME=/fake-home \
+                nix-instantiate "${flags[@]}" --eval --strict "lang/$i.nix" \
+                1> "lang/$i.out" \
+                2> "lang/$i.err"
+        then
+            sed -i "s!$(pwd)!/pwd!g" "lang/$i.out" "lang/$i.err"
+            diffAndAccept "$i" out exp
+            diffAndAccept "$i" err err.exp
+        else
+            echo "FAIL: $i should evaluate"
+            badExitCode=1
+        fi
+    fi
+done
+
+if test -n "${_NIX_TEST_ACCEPT-}"; then
+    if (( "$badDiff" )); then
+        echo 'Output did mot match, but accepted output as the persisted expected output.'
+        echo 'That means the next time the tests are run, they should pass.'
+    else
+        echo 'NOTE: Environment variable _NIX_TEST_ACCEPT is defined,'
+        echo 'indicating the unexpected output should be accepted as the expected output going forward,'
+        echo 'but no tests had unexpected output so there was no expected output to update.'
+    fi
+    if (( "$badExitCode" )); then
+        exit "$badExitCode"
+    else
+        skipTest "regenerating golden masters"
+    fi
+else
+    if (( "$badDiff" )); then
+        echo ''
+        echo 'You can rerun this test with:'
+        echo ''
+        echo '    _NIX_TEST_ACCEPT=1 make tests/functional/lang.sh.test'
+        echo ''
+        echo 'to regenerate the files containing the expected output,'
+        echo 'and then view the git diff to decide whether a change is'
+        echo 'good/intentional or bad/unintentional.'
+        echo 'If the diff contains arbitrary or impure information,'
+        echo 'please improve the normalization that the test applies to the output.'
+    fi
+    exit $(( "$badExitCode" + "$badDiff" ))
+fi