DiffRepository does not update log (_actual) file when assertion fails

Under normal circumstances when a test based on DiffRepository fails a log file ( with the _actual.xml suffix) is generated and contains the actual content from the test execution.

This DiffRepository feature is used to update the golden files easily by copying the log file to the respective file in resources.

Test fixtures such as RelOptFixture and SqlToRelFixture that rely upon DiffRepository are affected along with all tests using these.

DiffRepository and fixtures are published as part of the testkit module so consumers who are using the respective classes are now broken.