self resume, worker is instvar.
tearDown call moved to TestResult >> runCase:
RunningTestContext governs run of single test case.
Async SUnit tests, sans timeout.
tearDown called when async failure/error.
#async: block of finished test not run.
nextRunDo: removed, contextOf: creates context.
Async tests can now be rerun individually.
graceTime: now works and produces timeouts.
Merge pull request #285 from herby/async-tests

Asynchronous SUnit tests