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 68c81c737571794f7246db53fb4774e94fcf4b7e)

1 files changed, 0 insertions, 13 deletions

diff --git a/tests/lang/eval-okay-closure.nix b/tests/lang/eval-okay-closure.nix
deleted file mode 100644
index cccd4dc35..000000000
--- a/tests/lang/eval-okay-closure.nix
+++ /dev/null
@@ -1,13 +0,0 @@
-let
-
-  closure = builtins.genericClosure {
-    startSet = [{key = 80;}];
-    operator = {key, foo ? false}:
-      if builtins.lessThan key 0
-      then []
-      else [{key = builtins.sub key 9;} {key = builtins.sub key 13; foo = true;}];
-  };
-
-  sort = (import ./lib.nix).sortBy (a: b: builtins.lessThan a.key b.key);
-
-in sort closure