user32/tests: Add more SendMessageCallbackA/W() tests with NULL callback.
The tests are mainly to show that SendMessageCallbackA/W() with NULL callback pointer and callback
data being 1 causes the message to be posted to the window, even when the window is created in the
same thread.
win32u: Put the message in the queue when the callback pointer is NULL and the callback data is 1.
This is an undocumented behavior needed by React Native apps, based on Nikolay's findings.