netbsd-tests: Remove some pointless sleeps from message queue tests

• In the msgctl tests, there is no point in sleeping after a fork(). Just block immediately in wait().
• In non-blocking send/recv tests, just wait for the child to exit once it's reached a message limit. If a bug prevents the child from exiting promptly, the test will time out.

MFC after: 1 week

(cherry picked from commit 478de7f8e25849ce0b3a37b4baaf9c69e0b34072)