Follow-up to #1351: Restore TestFailure::failedTest()

Resolves #1351: Follow-up to restore backwards-compatibility for test Result printer implementations that rely on `TestFailure::failedTest()`.

Example: https://github.com/Codeception/Codeception/issues/1248

Requires #1360 to identify whether the failing test operates in process isolation.

Would you mind removing this since we've already merged #1360?