tests: Refactor tests of resizing animated images.
tests: Refactor tests of resizing animated images.

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