Add -e to exec single test in shell

[thinkerbot]

This is super-useful as a debugging tool. I've really enjoyed having it and miss it when it's gone. Current problems making this a work-in-progress:

• The exit status of the test is not reflected in the shell. This arises because currently -e does not use a true exec, instead it runs the test within the normal pipeline and simply turns off post-processing of the output. This is good for seeing the output but is a bit of a lie.
• No tests!