Merge ExecuteTest back into ExecuteTestInternalAsync

[campersau]

• Reduces the code duplication e.g. start and new TestResult. But method got slightly longer.
• Removes one try / catch / finally.
• Calculate duration and output for skipped tests just like done for the others.
• The EngineCancellationToken and its CancellationTokenSource were passed into SingleTestExecutor, now only EngineCancellationToken gets passed in.
• Only create another CancellationTokenSource when there are LinkedCancellationTokens.
• Renamed and reordered ExecuteTest* helper methods.