Add test for leaking children on errors

This tests the behavior fixed by 70f954821d52e564e483c95bab0ded0977e6b5c7.
Version 1.1.0

This version includes a potentially backwards incompatible change: when
passing Subprocess a file descriptor, it will now make a private copy
using #dup, meaning the file descriptor you pass will not be closed by
Subprocess.