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-xml.nix

@@ -0,0 +1,21 @@
+rec {
+
+  x = 123;
+
+  y = 567.890;
+
+  a = "foo";
+
+  b = "bar";
+
+  c = "foo" + "bar";
+
+  f = {z, x, y}: if y then x else z;
+
+  id = x: x;
+
+  at = args@{x, y, z}: x;
+
+  ellipsis = {x, y, z, ...}: x;
+
+}