winemac: When ordering sibling child windows, don't remove and re-add ones already in the right order.

winemac: Don't use Cocoa parent-child relationship when topmost state is enough to keep owned in front of owner.

The Cocoa parent-child relationship has undesirable side effects and bugs.  In
the general case, it's the only way to maintain the z-order of owned windows
relative to their owner.  But when the owner is non-topmost and an owned
window is topmost, the Cocoa window level will enforce that and we don't
need it.

kernel32/tests: Add a test to show that WaitCommEvent should succeed after SetCommMask(EV_TXEMPTY)+WriteFile+FlushFileBuffers.