[gdb/testsuite] Fix gdb.threads/thread-specific-bp.exp

On OBS I ran into a failure in test-case gdb.threads/thread-specific-bp.exp:
...
(gdb) PASS: gdb.threads/thread-specific-bp.exp: non-stop: continue to end
info breakpoint^M
Num     Type           Disp Enb Address            What^M
1       breakpoint     keep y   0x0000555555555167 in main at $src:36^M
        breakpoint already hit 1 time^M
2       breakpoint     keep y   0x0000555555555151 in start at $src:23^M
        breakpoint already hit 1 time^M
3       breakpoint     keep y   0x0000555555555167 in main at $src:36 thread 2^M
        stop only in thread 2^M
4       breakpoint     keep y   0x000055555555515c in end at $src:29^M
        breakpoint already hit 1 time^M
(gdb) [Thread 0x7ffff7db1640 (LWP 19984) 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: \
  thread-specific breakpoint was deleted (timeout)
...

Fix this by waiting for the "[Thread 0x7ffff7db1640 (LWP 19984) exited]"
message before issuing the "info breakpoint command".

Tested on x86_64-linux.