Merge #638

638: Make aio, chdir, and wait tests thread safe r=Susurrus

Fix thread safety issues in aio, chdir, and wait tests
    
They have four problems:
    
* The chdir tests change the process's cwd, which is global.  Protect them all with a mutex.
    
* The wait tests will reap any subprocess, and several tests create subprocesses.  Protect them all with a mutex so only one subprocess-creating test will run at a time.
    
* When a multithreaded test forks, the child process can sometimes block in the stack unwinding code.  It blocks on a mutex that was held by a different thread in the parent, but that thread doesn't exist in the child, so a deadlock results.  Fix this by immediately calling `std::process:;exit` in the child processes.
    
* My previous attempt at thread safety in the aio tests didn't work, because anonymous MutexGuards drop immediately.  Fix this by naming the SIGUSR2_MTX MutexGuards.

Fixes #251 

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