tests/server/util.[ch]: add exit event to stop waiting on Windows

tests/server/util.[ch]: add exit event to stop waiting on Windows

This commit adds a global exit event to the test servers that
Windows-specific wait routines can use to get triggered if the
program was signaled to be terminated, eg. select_ws in sockfilt.c

The exit event will be managed by the signal handling code and is
set to not reset automatically to support multiple wait routines.

Reviewed-by: Jay Satiro
Closes #5260