Rename the 'tools/functional/' directory to 'tests/functional/'

This makes more sense as these are functional tests and not tools.

This was dicussed in:
https://github.com/python-gitlab/python-gitlab/discussions/1468

Added a pytest.fixture for merge_request(). Use this fixture in
tools/functional/api/test_merge_requests.py

Have flake8 run at the top-level of the projects instead of just the
gitlab directory.

Have been seeing intermittent failures of the test_delete_user()
functional test. Have made the following changes to hopefully resolve
the issue and if it still fails to know better why the failure
occurred.

*  Extend the wait timeout for test_delete_user() from 30 to 60
   tries of 0.5 seconds each.

*  Modify wait_for_sidekiq() to return True if sidekiq process
   terminated. Return False if the timeout expired.

*  Modify wait_for_sidekiq() to loop through all processes instead of
   assuming there is only one process. If all processes are not busy
   then return.

*  Modify wait_for_sidekiq() to sleep at least once before checking
   for processes being busy.

*  Check for True being returned in test_delete_user() call to
   wait_for_sidekiq()