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, 52 insertions, 0 deletions

diff --git a/tests/functional/gc.sh b/tests/functional/gc.sh
new file mode 100644
index 000000000..ad09a8b39
--- /dev/null
+++ b/tests/functional/gc.sh
@@ -0,0 +1,52 @@
+source common.sh
+
+clearStore
+
+drvPath=$(nix-instantiate dependencies.nix)
+outPath=$(nix-store -rvv "$drvPath")
+
+# Set a GC root.
+rm -f "$NIX_STATE_DIR"/gcroots/foo
+ln -sf $outPath "$NIX_STATE_DIR"/gcroots/foo
+
+[ "$(nix-store -q --roots $outPath)" = "$NIX_STATE_DIR/gcroots/foo -> $outPath" ]
+
+nix-store --gc --print-roots | grep $outPath
+nix-store --gc --print-live | grep $outPath
+nix-store --gc --print-dead | grep $drvPath
+if nix-store --gc --print-dead | grep -E $outPath$; then false; fi
+
+nix-store --gc --print-dead
+
+inUse=$(readLink $outPath/reference-to-input-2)
+if nix-store --delete $inUse; then false; fi
+test -e $inUse
+
+if nix-store --delete $outPath; then false; fi
+test -e $outPath
+
+for i in $NIX_STORE_DIR/*; do
+    if [[ $i =~ /trash ]]; then continue; fi # compat with old daemon
+    touch $i.lock
+    touch $i.chroot
+done
+
+nix-collect-garbage
+
+# Check that the root and its dependencies haven't been deleted.
+cat $outPath/foobar
+cat $outPath/reference-to-input-2/bar
+
+# Check that the derivation has been GC'd.
+if test -e $drvPath; then false; fi
+
+rm "$NIX_STATE_DIR"/gcroots/foo
+
+nix-collect-garbage
+
+# Check that the output has been GC'd.
+if test -e $outPath/foobar; then false; fi
+
+# Check that the store is empty.
+rmdir $NIX_STORE_DIR/.links
+rmdir $NIX_STORE_DIR