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, 34 insertions, 0 deletions

diff --git a/tests/functional/pure-eval.sh b/tests/functional/pure-eval.sh
new file mode 100644
index 000000000..5334bf28e
--- /dev/null
+++ b/tests/functional/pure-eval.sh
@@ -0,0 +1,34 @@
+source common.sh
+
+clearStore
+
+nix eval --expr 'assert 1 + 2 == 3; true'
+
+[[ $(nix eval --impure --expr 'builtins.readFile ./pure-eval.sh') =~ clearStore ]]
+
+missingImpureErrorMsg=$(! nix eval --expr 'builtins.readFile ./pure-eval.sh' 2>&1)
+
+echo "$missingImpureErrorMsg" | grepQuiet -- --impure || \
+    fail "The error message should mention the “--impure” flag to unblock users"
+
+[[ $(nix eval --expr 'builtins.pathExists ./pure-eval.sh') == false ]] || \
+    fail "Calling 'pathExists' on a non-authorised path should return false"
+
+(! nix eval --expr builtins.currentTime)
+(! nix eval --expr builtins.currentSystem)
+
+(! nix-instantiate --pure-eval ./simple.nix)
+
+[[ $(nix eval --impure --expr "(import (builtins.fetchurl { url = file://$(pwd)/pure-eval.nix; })).x") == 123 ]]
+(! nix eval --expr "(import (builtins.fetchurl { url = file://$(pwd)/pure-eval.nix; })).x")
+nix eval --expr "(import (builtins.fetchurl { url = file://$(pwd)/pure-eval.nix; sha256 = \"$(nix hash file pure-eval.nix --type sha256)\"; })).x"
+
+rm -rf $TEST_ROOT/eval-out
+nix eval --store dummy:// --write-to $TEST_ROOT/eval-out --expr '{ x = "foo" + "bar"; y = { z = "bla"; }; }'
+[[ $(cat $TEST_ROOT/eval-out/x) = foobar ]]
+[[ $(cat $TEST_ROOT/eval-out/y/z) = bla ]]
+
+rm -rf $TEST_ROOT/eval-out
+(! nix eval --store dummy:// --write-to $TEST_ROOT/eval-out --expr '{ "." = "bla"; }')
+
+(! nix eval --expr '~/foo')