Fix signal-while-stepping-over-bp-other-thread.exp on targets always in non-stop

Fix signal-while-stepping-over-bp-other-thread.exp on targets always in non-stop

With "maint set target-non-stop on" we get:

 -PASS: gdb.threads/signal-while-stepping-over-bp-other-thread.exp: step
 +FAIL: gdb.threads/signal-while-stepping-over-bp-other-thread.exp: step

The issue is simply that switch_back_to_stepped_thread is not used in
non-stop mode, thus infrun doesn't output the expected "switching back
to stepped thread" log.

gdb/testsuite/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* signal-while-stepping-over-bp-other-thread.exp: Expect "restart
	threads" as alternative to "switching back to stepped thread".