Allow a test to identify whether it runs in isolation.

A custom setup of test fixtures may use this indicator to skip additional performance optimizations that only make sense if multiple tests are executed in the same process.
Merge pull request #1360 from sun/istestinisolation

Allow a test to identify whether it runs in isolation
Fix #1351: Add back TestFailure::failedTest() for BC.
Merge pull request #1359 from sun/result-isolation-bc

Follow-up to #1351: Restore TestFailure::failedTest()
Fixed versions numbers from 6529e40 (refs #1352).