Reduce amount of initial connects during non-SQLite test-RDBMS lock-grabs

Reduce amount of initial connects during non-SQLite test-RDBMS lock-grabs

I am not sure why I wrote things this way back in 8d6b1478d, perhaps I didn't
properly think through that sqlt_type will be correctly determined no matter
what. As a result any MySQL test would connect at least once, and under the
right circumstances consume enough per-connection-thread memory to get a
low-RAM system (like TravisCI) to start OOM-killing things indiscriminately.