Add tests for `wait`

[sunfishcode]

As discussed in https://github.com/bytecodealliance/rustix/pull/165, wait is a tricky function to test from within the test harness because it depends on knowing that nothing else in the process is creating child processes which the wait might wait for. To test it, we should ideally create a new process, and have the new process create a new process and wait for it, so we can be sure of what it's waiting for.