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, 1 insertions, 1 deletions

diff --git a/.github/labeler.yml b/.github/labeler.yml
index 12120bdb3..7544f07a6 100644
--- a/.github/labeler.yml
+++ b/.github/labeler.yml
@@ -20,4 +20,4 @@
   # Unit tests
   - src/*/tests/**/*
   # Functional and integration tests
-  - tests/**/*
+  - tests/functional/**/*