tests: Add assertLogs for test_auth_backends.
tests: Add assertLogs for test_auth_backends.

This commit tests logging of warning log using assertLogs instead of
printing it in test output hence avoiding spam in test output.

punchagan/zulip,showell/zulip,punchagan/zulip,brainwane/zulip,hackerkid/zulip,eeshangarg/zulip,brainwane/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,rht/zulip,zulip/zulip,kou/zulip,kou/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,showell/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,rht/zulip,kou/zulip,showell/zulip,showell/zulip,punchagan/zulip,zulip/zulip,brainwane/zulip,zulip/zulip,showell/zulip,brainwane/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,rht/zulip,punchagan/zulip,andersk/zulip,rht/zulip,andersk/zulip,rht/zulip,showell/zulip,rht/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,showell/zulip,andersk/zulip,brainwane/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip