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, 11 deletions

diff --git a/tests/lang/eval-okay-listtoattrs.nix b/tests/lang/eval-okay-listtoattrs.nix
deleted file mode 100644
index 4186e029b..000000000
--- a/tests/lang/eval-okay-listtoattrs.nix
+++ /dev/null
@@ -1,11 +0,0 @@
-# this test shows how to use listToAttrs and that evaluation is still lazy (throw isn't called)
-with import ./lib.nix;
-
-let 
-  asi = name: value : { inherit name value; };
-  list = [ ( asi "a" "A" ) ( asi "b" "B" ) ];
-  a = builtins.listToAttrs list;
-  b = builtins.listToAttrs ( list ++ list );
-  r = builtins.listToAttrs [ (asi "result" [ a b ]) ( asi "throw" (throw "this should not be thrown")) ];
-  x = builtins.listToAttrs [ (asi "foo" "bar") (asi "foo" "bla") ];
-in concat (map (x: x.a) r.result) + x.foo