tests: Add a mock to verify that queue_json_publish is valid JSON.
tests: Add a mock to verify that queue_json_publish is valid JSON.

See also #16121.  However, that does not address the tests that
stub out the call to queue_json_publish itself.

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