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, 6 insertions, 2 deletions

diff --git a/mk/common-test.sh b/mk/common-test.sh
index 0a2e4c1c2..7ab25febf 100644
--- a/mk/common-test.sh
+++ b/mk/common-test.sh
@@ -1,11 +1,15 @@
+test_dir=tests/functional
+
+test=$(echo -n "$test" | sed -e "s|^$test_dir/||")
+
 TESTS_ENVIRONMENT=("TEST_NAME=${test%.*}" 'NIX_REMOTE=')
 
 : ${BASH:=/usr/bin/env bash}
 
 init_test () {
-   cd tests && env "${TESTS_ENVIRONMENT[@]}" $BASH -e init.sh 2>/dev/null > /dev/null
+   cd "$test_dir" && env "${TESTS_ENVIRONMENT[@]}" $BASH -e init.sh 2>/dev/null > /dev/null
 }
 
 run_test_proper () {
-   cd $(dirname $test) && env "${TESTS_ENVIRONMENT[@]}" $BASH -e $(basename $test)
+   cd "$test_dir/$(dirname $test)" && env "${TESTS_ENVIRONMENT[@]}" $BASH -e $(basename $test)
 }