Make the checkins_interval transient datasets more uniquely named in BigQuery test runs (#18600)

* Make the checkins_interval transient datasets more uniquely named

Append a timestamp suffix (initialized when the namespace was loaded) to transient dataset names

Add verbose comments explaining what we're doing

Add transient-dataset? functions for both BQ drivers to put logic in one place (which currently just see if the dataset is named `checkins_interval_*`

Ditch the "legacydriver_" prefix from the legacy BigQuery driver when creating new datasets; it's no longer needed now that transient datasets will have a much more unique suffix

Update the logic for the transient dataset suffix so that it can be more easily detected/parsed later (for cleanup)

Remove old (more than 2 hours) transient datasets when test extensions namespaces load

Add various docstrings