Previously the only option was to create a pipe or an ipc channel. This
patch makes it possible to inherit a handle that is already open in the
parent process. There is also room for setting more than just stdin,
stdout and stderr, although this is not supported yet.
If a test failed we would previously see:
Output from process `test_foo`: blah
Output from process `test_foo`: (nothing)
This commit changes it to:
Output from process `test_foo`: blah
Output from process `foo_helper`: (nothing)
