[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.

gdb/testsuite/gdb.threads/thread-specific-bp.exp

@@ -87,9 +87,24 @@ proc check_thread_specific_breakpoint {mode} {
 	    set cmd "continue"
 	}
 	set test "continue to end"
-	gdb_test_multiple "$cmd" $test {
-	    -re "Breakpoint .* end .* at .*\r\n$gdb_prompt " {
-		pass $test
+	set thread_exited 0
+	set prompt 0
+	gdb_test_multiple "$cmd" $test -lbl {
+	    -re "(^|\r\n)\\\[Thread \[^\r\n\]* exited](?=\r\n)" {
+		if { $prompt } {
+		    pass $gdb_test_name
+		} else {
+		    set thread_exited 1
+		    exp_continue
+		}
+	    }
+	    -re "\r\n$gdb_prompt " {
+		if { $thread_exited } {
+		    pass $gdb_test_name
+		} else {
+		    set prompt 1
+		    exp_continue
+		}
 	    }
 	}