Minor updates to output-per-test-case spec.
Add stories for supporting parallel execution of test cases within the same vm (at method and class level).

Adding as this is said to be requirement of the story for supporting output-per-test case in the XML report. More specifically, that we won't support output-per-test case until we support running more than one test case at one time in a JVM .

The implementation details of these specs is TBD at the moment. Working out how to actually support this is non trivial, so I'm deferring spec'ing the implementation until we have an idea of when we are scheduling it.