Allow clean abort

[snogge]

Originally reported as PR #167 by @doublep .

Add an option to immediately abort the test run if a test fails.

This is useful if your tests are so interdependent (at least in the context of the current bug) that one failure typically means there will be 20 more. Also when tests are slow enough that you don't want to wait for all of them to finish to get the backtrace of the first failure.