Add test for user context selection sync

Add test for user context selection sync

This patch adds a test to verify that events are sent properly to all
UIs when the user selection context (inferior, thread, frame) changes.

The goal of the C test file is to provide two threads that are stopped with the
same predictable backtrace (so that we can test frame switching).  The barrier
helps us know when the child threads are started.  Then, scheduler-locking is
used to bring each thread one by one to the position we expect them to be
during the test.

gdb/testsuite/ChangeLog:

YYYY-MM-DD  Antoine Tremblay  <antoine.tremblay@ericsson.com>
YYYY-MM-DD  Simon Marchi  <simon.marchi@ericsson.com>

	PR gdb/20487
	* gdb.mi/user-selected-context-sync.exp: New file.
	* gdb.mi/user-selected-context-sync.c: New file.