test_queue: Use assertLogs to verify warning in test_queue_error_json.
test_queue: Use assertLogs to verify warning in test_queue_error_json.

This will avoid spam in test-backend output.

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