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-fail-substring.err.exp

@@ -0,0 +1,10 @@
+error:
+       … while calling the 'substring' builtin
+
+         at /pwd/lang/eval-fail-substring.nix:1:1:
+
+            1| builtins.substring (builtins.sub 0 1) 1 "x"
+             | ^
+            2|
+
+       error: negative start position in 'substring'