Use database independent calls to initialize the testing cache.

Describe your issue or idea

The testing cache is initialized with a MySQL-specific SHOW TABLES call and a couple db_query()s to close the tables. This SQL can be refactored into the MySQL database classes.

Steps to reproduce (if reporting a bug)

1. Fork the project
2. Restore the pgsql database classes
3. Try to get the tests to work using pgsql with caching

(Yeah, not exactly an "80% of sites" use case, is it? ;) )

Relevant version/system information (if applicable)

Backdrop 1.8 / Silkscreen 1.7.x