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-regex-match.nix

@@ -0,0 +1,29 @@
+with builtins;
+
+let
+
+  matches = pat: s: match pat s != null;
+
+  splitFN = match "((.*)/)?([^/]*)\\.(nix|cc)";
+
+in
+
+assert  matches "foobar" "foobar";
+assert  matches "fo*" "f";
+assert !matches "fo+" "f";
+assert  matches "fo*" "fo";
+assert  matches "fo*" "foo";
+assert  matches "fo+" "foo";
+assert  matches "fo{1,2}" "foo";
+assert !matches "fo{1,2}" "fooo";
+assert !matches "fo*" "foobar";
+assert  matches "[[:space:]]+([^[:space:]]+)[[:space:]]+" "  foo   ";
+assert !matches "[[:space:]]+([[:upper:]]+)[[:space:]]+" "  foo   ";
+
+assert match "(.*)\\.nix" "foobar.nix" == [ "foobar" ];
+assert match "[[:space:]]+([[:upper:]]+)[[:space:]]+" "  FOO   " == [ "FOO" ];
+
+assert splitFN "/path/to/foobar.nix" == [ "/path/to/" "/path/to" "foobar" "nix" ];
+assert splitFN "foobar.cc" == [ null null "foobar" "cc" ];
+
+true