Fix gdb.threads/non-ldr-exc-3.exp race

Fix gdb.threads/non-ldr-exc-3.exp race

gdb.threads/non-ldr-exc-3.exp is sometimes failing like this:

 [Switching to Thread 6831.6832]

 Breakpoint 2, thread_execler (arg=0x0) at /home/pedro/gdb/mygit/build/../src/gdb/testsuite/gdb.threads/non-ldr-exc-3.c:41
 41        if (execl (image, image, argv1, NULL) == -1) /* break-here */
 PASS: gdb.threads/non-ldr-exc-3.exp: lock-sched=on,non-stop=off: continue to breakpoint
 (gdb) set scheduler-locking on
 (gdb) FAIL: gdb.threads/non-ldr-exc-3.exp: lock-sched=on,non-stop=off: set scheduler-locking on

The problem is that the gdb_test_multiple is missing the prompt
anchor.  The problem was introduced by 2fd33e9448.  This reverts the
hunk that introduced the problem, reverting back to
gdb_continue_to_breakpoint.

gdb/testsuite/ChangeLog:
2015-09-15  Pedro Alves  <palves@redhat.com>

	* gdb.threads/non-ldr-exc-3.exp (do_test): Use
	gdb_continue_to_breakpoint instead of gdb_test_multiple.