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, 53 insertions, 0 deletions

diff --git a/tests/functional/check-refs.sh b/tests/functional/check-refs.sh
new file mode 100644
index 000000000..3b587d1e5
--- /dev/null
+++ b/tests/functional/check-refs.sh
@@ -0,0 +1,53 @@
+source common.sh
+
+clearStore
+
+RESULT=$TEST_ROOT/result
+
+dep=$(nix-build -o $RESULT check-refs.nix -A dep)
+
+# test1 references dep, not itself.
+test1=$(nix-build -o $RESULT check-refs.nix -A test1)
+nix-store -q --references $test1 | grepQuietInverse $test1
+nix-store -q --references $test1 | grepQuiet $dep
+
+# test2 references src, not itself nor dep.
+test2=$(nix-build -o $RESULT check-refs.nix -A test2)
+nix-store -q --references $test2 | grepQuietInverse $test2
+nix-store -q --references $test2 | grepQuietInverse $dep
+nix-store -q --references $test2 | grepQuiet aux-ref
+
+# test3 should fail (unallowed ref).
+(! nix-build -o $RESULT check-refs.nix -A test3)
+
+# test4 should succeed.
+nix-build -o $RESULT check-refs.nix -A test4
+
+# test5 should succeed.
+nix-build -o $RESULT check-refs.nix -A test5
+
+# test6 should fail (unallowed self-ref).
+(! nix-build -o $RESULT check-refs.nix -A test6)
+
+# test7 should succeed (allowed self-ref).
+nix-build -o $RESULT check-refs.nix -A test7
+
+# test8 should fail (toFile depending on derivation output).
+(! nix-build -o $RESULT check-refs.nix -A test8)
+
+# test9 should fail (disallowed reference).
+(! nix-build -o $RESULT check-refs.nix -A test9)
+
+# test10 should succeed (no disallowed references).
+nix-build -o $RESULT check-refs.nix -A test10
+
+if isDaemonNewer 2.12pre20230103; then
+    if ! isDaemonNewer 2.16.0; then
+        enableFeatures discard-references
+        restartDaemon
+    fi
+
+    # test11 should succeed.
+    test11=$(nix-build -o $RESULT check-refs.nix -A test11)
+    [[ -z $(nix-store -q --references "$test11") ]]
+fi