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/parallel.builder.sh

@@ -0,0 +1,29 @@
+echo "DOING $text"
+
+
+# increase counter
+while ! ln -s x $shared.lock 2> /dev/null; do
+    sleep 1
+done
+test -f $shared.cur || echo 0 > $shared.cur
+test -f $shared.max || echo 0 > $shared.max
+new=$(($(cat $shared.cur) + 1))
+if test $new -gt $(cat $shared.max); then
+    echo $new > $shared.max
+fi
+echo $new > $shared.cur
+rm $shared.lock
+
+
+echo -n $(cat $inputs)$text > $out
+
+sleep $sleepTime
+
+
+# decrease counter
+while ! ln -s x $shared.lock 2> /dev/null; do
+    sleep 1
+done
+test -f $shared.cur || echo 0 > $shared.cur
+echo $(($(cat $shared.cur) - 1)) > $shared.cur
+rm $shared.lock