tests: add CFRunLoop mode membership tests

[DTW-Thalion]

Adds coverage for the CFRunLoop mode-membership surface, exercised on a fresh run loop without running it:

• CFRunLoopGetCurrent / CFRunLoopGetMain return a run loop
• CFRunLoopCopyCurrentMode is NULL when the loop is not running
• CFRunLoopCopyAllModes returns a non-NULL array and lists a mode once a source is added to it
• CFRunLoopAddSource/AddObserver/AddTimer, CFRunLoopContainsSource/ContainsObserver/ContainsTimer and CFRunLoopRemoveSource/RemoveObserver/RemoveTimer round-trip per mode
• a source added through kCFRunLoopCommonModes is reported as contained in kCFRunLoopCommonModes, kCFRunLoopDefaultMode and a registered common mode

Verified against Apple CoreFoundation (21 assertions, no divergence).