[Bug 54037] user32:msg - test_SendMessage_other_thread() sometimes gets a todo success (GitLab CI)

[WineHQ Bugzilla]

https://bugs.winehq.org/show_bug.cgi?id=54037

--- Comment #2 from Esme Povirk  ---
Documenting so I don't have to figure this out again:

There's an obvious race condition the SendMessage_thread_2 case. The thread
calls SetParent immediately before a SendMessage. The expectation is that on
Wine the foreground thread will wait for QS_SENDMESSAGE, wake (incorrectly)
after the SetParent call, and then process one posted message from the queue,
leaving the background thread stuck in its SendMessage call. However, if the
background thread's SendMessage call happens before the foreground thread
processes the posted message, the foreground thread will handle both posted
messages in a single GetMessage call.

I don't think this test case is appropriate for testing that SetParent doesn't
use an internal message on Windows.