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, 21 deletions

diff --git a/tests/lang/eval-okay-substring.nix b/tests/lang/eval-okay-substring.nix
deleted file mode 100644
index 424af00d9..000000000
--- a/tests/lang/eval-okay-substring.nix
+++ /dev/null
@@ -1,21 +0,0 @@
-with builtins;
-
-let
-
-  s = "foobar";
-
-in
-
-substring 1 2 s
-+ "x"
-+ substring 0 (stringLength s) s
-+ "y"
-+ substring 3 100 s
-+ "z"
-+ substring 2 (sub (stringLength s) 3) s
-+ "a"
-+ substring 3 0 s
-+ "b"
-+ substring 3 1 s
-+ "c"
-+ substring 5 10 "perl"