tests: introduce REUSE_TEST_DB as an alternative to TEST_DB

With TEST_DB, the specified database would be dropped (if existing) and created
from scratch. That would fail if the user only had been granted access to that
database without general database creation permissions.

With REUSE_TEST_DB, the database must exist, and the tables in the existing
database will be deleted before creating the new ones.

This is for example useful for testing on PostgreSQL without full database
privileges. (With MariaDB/MySQL, it is possible to grant create permissions to
a database before it exists, so it is easier to use TEST_DB.)