Fix functional tests

[sea-kelp]

Description of Changes

Bug:

• pytest 6.2.0 introduced a change that warns about unhandled thread exceptions, which likely means that the tests were erroring before we upgraded and just weren't aware of it: https://docs.pytest.org/en/7.1.x/changelog.html#id129
• Currently, we try to create a new flask thread for each functional test and since they all try to use the same port, all but the first thread to throws this error:

    warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))

tests/test_functional.py::test_officer_form_has_units_alpha_sorted
  /usr/local/lib/python3.10/site-packages/_pytest/threadexception.py:75: PytestUnhandledThreadExceptionWarning: Exception in thread Thread-295 (run)
  
  Traceback (most recent call last):
    File "/usr/local/lib/python3.10/site-packages/werkzeug/serving.py", line 911, in prepare_socket
      s.bind(server_address)
  OSError: [Errno 98] Address already in use
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/usr/local/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
      self.run()
    File "/usr/local/lib/python3.10/threading.py", line 953, in run
      self._target(*self._args, **self._kwargs)
    File "/usr/local/lib/python3.10/site-packages/flask/app.py", line 920, in run
      run_simple(t.cast(str, host), port, self, **options)
    File "/usr/local/lib/python3.10/site-packages/werkzeug/serving.py", line 1062, in run_simple
      s = prepare_socket(hostname, port)
    File "/usr/local/lib/python3.10/site-packages/werkzeug/serving.py", line 930, in prepare_socket
      sys.exit(1)
  SystemExit: 1

Change:

• Set flask app fixture scope to "session" so we don't try to run multiple instances on the same port

Notes for Deployment

None!

Screenshots (if appropriate)

N/A

Tests and linting

• I have rebased my changes on main

• just lint passes

• just test passes

[AetherUnbound]

Wow, what a find! Thank you!