Expand event loop post-dispatch-check tests
See !45 (comment 387678) . It could be helpful to verify that the dispatch functions for timer, idle, and signal event sources still behave correctly when marked for re-checking. Still more edge cases might be found by having the callbacks for the source modify other sources.
(At the moment, it looks like event sources added by
wl_event_loop_add_idle don't have a dispatch handler, and are added to the
loop->idle_list list on construction, so trying to call
wl_event_source_check will probably cause a crash.)
Of course, it is possible that nobody actually uses
wl_event_source_check with anything other than file descriptor event sources; in that case, it may suffice to adjust the documentation and then close this issue.