tweaks to the AtFork tests
tweaks to the AtFork tests

Summary: Use `waitpid` for more exact waits. Use `_exit` in the child to skip unnecessary teardown work. Remove unused local variable.

Differential Revision: D34191704

fbshipit-source-id: 38f4275eab38a4ba5a41fbf617aa3fb389ea00c0

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly