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/src/libutil/url-parts.hh b/src/libutil/url-parts.hh
index 98162b0f7..12026921f 100644
--- a/src/libutil/url-parts.hh
+++ b/src/libutil/url-parts.hh
@@ -30,7 +30,7 @@ extern std::regex refRegex;
 
 /// Instead of defining what a good Git Ref is, we define what a bad Git Ref is
 /// This is because of the definition of a ref in refs.c in https://github.com/git/git
-/// See tests/fetchGitRefs.sh for the full definition
+/// See tests/functional/fetchGitRefs.sh for the full definition
 const static std::string badGitRefRegexS = "//|^[./]|/\\.|\\.\\.|[[:cntrl:][:space:]:?^~\[]|\\\\|\\*|\\.lock$|\\.lock/|@\\{|[/.]$|^@$|^$";
 extern std::regex badGitRefRegex;