Improve complex order+prefetch exception message
Move the DSN-lock machinery from 8d6b1478d into DBICTest::BaseSchema

Just a c/p job, zero functional changes
This test was essentially c/p-ed in 2cc3a7be3, consolidate

The overriding difference is the new-connection == new-storage == new-sqla
Add an explicit codepath to test that, but overall there is plenty of
other places that verify this behavior to death
Reaching for IO::File here makes no sense
Consolidate pg/mysql and standalone SQLite savepoint tests to run on all 3

A larger consolidation will take place later on with T::WV, small steps