1. Fix drivers.t so it passes when no DB drivers are available.
2. Make it possible for the test suite to detect if certain DB drivers
are available, so individual tests that require them can be skipped.
If a test did not support intermediate reports (i.e. --report-interval
or --report-checkpoints), there was a confusing (but benign) error at
the end of the test: "Terminating the reporting thread failed".