selftests/coredump: Fix "socket_detect_userspace_client" test failure

The coredump.socket_detect_userspace_client test occasionally fails:
    #  RUN           coredump.socket_detect_userspace_client ...
    # stackdump_test.c:500:socket_detect_userspace_client:Expected 0 (0) != WIFEXITED(status) (0)
    # socket_detect_userspace_client: Test terminated by assertion
    #          FAIL  coredump.socket_detect_userspace_client
    not ok 3 coredump.socket_detect_userspace_client

because there is no guarantee that client's write() happens before server's
close(). The client gets terminated SIGPIPE, and thus the test fails.

Add a read() to server to make sure server's close() doesn't happen before
client's write().

Fixes: 7b6724f ("selftests/coredump: add tests for AF_UNIX coredumps")
Signed-off-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/20250620110252.1640391-1-namcao@linutronix.de
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: covanam

tools/testing/selftests/coredump/stackdump_test.c

@@ -461,10 +461,15 @@ TEST_F(coredump, socket_detect_userspace_client)
 			_exit(EXIT_FAILURE);
 		}
 
+		ret = read(fd_coredump, &c, 1);
+
 		close(fd_coredump);
 		close(fd_server);
 		close(fd_peer_pidfd);
 		close(fd_core_file);
+
+		if (ret < 1)
+			_exit(EXIT_FAILURE);
 		_exit(EXIT_SUCCESS);
 	}
 	self->pid_coredump_server = pid_coredump_server;