Fix racy test in gdb.threads/thread-specific-bp.exp

Fix racy test in gdb.threads/thread-specific-bp.exp

I see the following test fail from time to time, due to the racy test
in gdb.threads/thread-specific-bp.exp.

continue -a^M
Continuing.^M
^M
Thread 1 "thread-specific" hit Breakpoint 4, end () at binutils-gdb/gdb/testsuite/gdb.threads/thread-specific-bp.c:29^M
29      }^M
(gdb) [Thread 0x40322460 (LWP 12950) exited]^M
Thread-specific breakpoint 3 deleted - thread 2 no longer in the thread list.^M
FAIL: gdb.threads/thread-specific-bp.exp: non-stop: continue to end (timeout)

This patch changes gdb_test to gdb_test_multiple to match prompt only
instead of both prompt and anchor.

gdb/testsuite:

2017-04-05  Yao Qi  <yao.qi@linaro.org>

	* gdb.threads/thread-specific-bp.exp (check_thread_specific_breakpoint):
	Use gdb_test_multiple, and don't match anchor.