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/lang/eval-okay-comments.nix

@@ -0,0 +1,59 @@
+# A simple comment
+"a"+ # And another
+## A double comment
+"b"+  ## And another
+# Nested # comments #
+"c"+   # and # some # other #
+# An empty line, following here:
+
+"d"+      # and a comment not starting the line !
+
+"e"+
+/* multiline comments */
+"f" +
+/* multiline
+   comments,
+   on
+   multiple
+   lines
+*/
+"g" +
+# Small, tricky comments
+/**/ "h"+ /*/*/ "i"+ /***/ "j"+ /* /*/ "k"+ /*/* /*/ "l"+
+# Comments with an even number of ending '*' used to fail:
+"m"+
+/* */ /* **/ /* ***/ /* ****/ "n"+
+/* */ /** */ /*** */ /**** */ "o"+
+/** **/ /*** ***/ /**** ****/ "p"+
+/* * ** *** **** ***** */     "q"+
+# Random comments
+/* ***** ////// * / * / /* */ "r"+
+# Mixed comments
+/* # */
+"s"+
+# /* #
+"t"+
+# /* # */
+"u"+
+# /*********/
+"v"+
+## */*
+"w"+
+/*
+ * Multiline, decorated comments
+ * # This ain't a nest'd comm'nt
+ */
+"x"+
+''${/** with **/"y"
+  # real
+  /* comments
+     inside ! # */
+
+  # (and empty lines)
+
+}''+          /* And a multiline comment,
+                 on the same line,
+                 after some spaces
+*/             # followed by a one-line comment
+"z"
+/* EOF */