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/gc-runtime.sh

@@ -0,0 +1,38 @@
+source common.sh
+
+case $system in
+    *linux*)
+        ;;
+    *)
+        skipTest "Not running Linux";
+esac
+
+set -m # enable job control, needed for kill
+
+profiles="$NIX_STATE_DIR"/profiles
+rm -rf $profiles
+
+nix-env -p $profiles/test -f ./gc-runtime.nix -i gc-runtime
+
+outPath=$(nix-env -p $profiles/test -q --no-name --out-path gc-runtime)
+echo $outPath
+
+echo "backgrounding program..."
+$profiles/test/program &
+sleep 2 # hack - wait for the program to get started
+child=$!
+echo PID=$child
+
+nix-env -p $profiles/test -e gc-runtime
+nix-env -p $profiles/test --delete-generations old
+
+nix-store --gc
+
+kill -- -$child
+
+if ! test -e $outPath; then
+    echo "running program was garbage collected!"
+    exit 1
+fi
+
+exit 0