Change asyncio mode to auto.

func tests for juju 2.9/focal were failing. This was because the asyncio
marker was not added to the tests.

This change adds the markers automatically to all the tests by passing
a CLI argument.

Refer:
https://pytest-asyncio.readthedocs.io/en/latest/concepts.html#test-discovery-modes
https://pytest-asyncio.readthedocs.io/en/latest/reference/configuration.html

tox.ini

@@ -77,12 +77,12 @@ deps =
     pytest-cov
 
 [testenv:func]
-commands = pytest {toxinidir}/tests/functional {posargs:-v}
+commands = pytest {toxinidir}/tests/functional --asyncio-mode=auto {posargs:-v}
 deps =
   -r {toxinidir}/tests/functional/requirements.txt
   juju < 3
 
 [testenv:func31]
-commands = pytest {toxinidir}/tests/functional {posargs:-v}
+commands = pytest {toxinidir}/tests/functional --asyncio-mode=auto {posargs:-v}
 deps =
   -r {toxinidir}/tests/functional/requirements.txt