add non-stop test that stresses thread starvation issues

add non-stop test that stresses thread starvation issues

This commit adds a non-stop mode test originally inspired by
signal-while-stepping-over-bp-other-thread.exp, that exposes the
thread starvation issues fixed by the previous patches.  It sets a set
of threads stepping in parallel, and has one of them get a signal.
Without the previous fixes, this would fail with timeouts.

gdb/testsuite/
2015-01-09  Pedro Alves  <palves@redhat.com>

	* gdb.threads/non-stop-fair-events.c: New file.
	* gdb.threads/non-stop-fair-events.exp: New file.