tests: Mark test_migrations tests to be automatically skipped.
tests: Mark test_migrations tests to be automatically skipped.

This fixes an issue where these tests will fail when adding new
migrations on top of them.

andersk/zulip,kou/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,rht/zulip,kou/zulip,zulip/zulip,zulip/zulip,andersk/zulip,kou/zulip,kou/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,kou/zulip,andersk/zulip,zulip/zulip,rht/zulip