- given that Oracle's issue is not just open connections, but *closed* ones too,...

- given that Oracle's issue is not just open connections, but *closed* ones too, go really
heavy handed and start marking tests as "requires.ad_hoc_engines", add a flag --low-connections
that will switch the engine reaper mechanism to use as *few* distinct engines and connections
as possible, many engine tests that really need their own engines are just skipped.