Allow for mid-flight execution stopping

This requires a few things. The basic one is to be able and stop the TestRunner, which is sort of the easy part.
The second might be to be able and stop the TestExecutor, and that's... complicated. Tests are supposed to be easy to setup, this would make them harder to create and handle. Ideally they should also be short live.
The third item is to stop whatever is driving the test run, that is, whatever is creating the test runners and executing them. In the junit case, that's the junit testrunner, Eclipse, or maven... That is, some place where DuckHawk is not in control. We could have the integration just skip the tests in that case.