We have new parameters "how many parallel tests this instance can run"
( test-runner.node-count ) and "how many supplementary databases each
test needs" ( test-runner.extra-database-count ).

Instead of configuring "I need a total of 15 databases", we configure "I
need to be able to run 3 tests in parallel, each test requiring 4 extra
databases" which generates 3 * (4 +1) databases.

`mariadb.test-database-amount` becomes deprecated in favour of these new
arguments, but compatibility is preserved.