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/
```

(cherry picked from commit 68c81c7375)

tests/functional/lang/eval-fail-hashfile-missing.err.exp

@@ -0,0 +1,19 @@
+error:
+       … while calling the 'toString' builtin
+
+         at /pwd/lang/eval-fail-hashfile-missing.nix:4:3:
+
+            3| in
+            4|   toString (builtins.concatLists (map (hash: map (builtins.hashFile hash) paths) ["md5" "sha1" "sha256" "sha512"]))
+             |   ^
+            5|
+
+       … while evaluating the first argument passed to builtins.toString
+
+         at «none»:0: (source not available)
+
+       … while calling the 'hashFile' builtin
+
+         at «none»:0: (source not available)
+
+       error: opening file '/pwd/lang/this-file-is-definitely-not-there-7392097': No such file or directory