tests: Improve authentication_methods schema.
tests: Improve authentication_methods schema.

This also removes the last use of the "loose"
check_dict() validator in the events tests.

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