Mock system(), exec(), qx() (AKA `` and readpipe()) with your own
functions in order to test code that may call them.

Some uses might be:

- avoid actually running the system command, just pretend we did
  (simulate [un]expected output, return values, etc)
- test various return value handling (e.g. the system command core dumps
  how does the object handle that)
- test that the arguments that will be passed to a system command
  are correct
- simulate that really hard to reproduce low level edge case to make
  sure your code works correctly on affected systems
- etc etc