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/
```

tests/functional/nix-collect-garbage-d.sh

@@ -0,0 +1,40 @@
+source common.sh
+
+clearStore
+
+## Test `nix-collect-garbage -d`
+
+# TODO make `nix-env` doesn't work with CA derivations, and make
+# `ca/nix-collect-garbage-d.sh` wrapper.
+
+testCollectGarbageD () {
+    clearProfiles
+    # Run two `nix-env` commands, should create two generations of
+    # the profile
+    nix-env -f ./user-envs.nix -i foo-1.0 "$@"
+    nix-env -f ./user-envs.nix -i foo-2.0pre1 "$@"
+    [[ $(nix-env --list-generations "$@" | wc -l) -eq 2 ]]
+
+    # Clear the profile history. There should be only one generation
+    # left
+    nix-collect-garbage -d
+    [[ $(nix-env --list-generations "$@" | wc -l) -eq 1 ]]
+}
+
+testCollectGarbageD
+
+# Run the same test, but forcing the profiles an arbitrary location.
+rm ~/.nix-profile
+ln -s $TEST_ROOT/blah ~/.nix-profile
+testCollectGarbageD
+
+# Run the same test, but forcing the profiles at their legacy location under
+# /nix/var/nix.
+#
+# Note that we *don't* use the default profile; `nix-collect-garbage` will
+# need to check the legacy conditional unconditionally not just follow
+# `~/.nix-profile` to pass this test.
+#
+# Regression test for #8294
+rm ~/.nix-profile
+testCollectGarbageD --profile "$NIX_STATE_DIR/profiles/per-user/me"