Change all `MiniTest` to `Minitest` since, `MiniTest` namespace has been renamed to `Minitest`
Ref: https://github.com/seattlerb/minitest/blob/master/History.txt
Merge pull request #13366 from vipulnsward/minitest-namespace-change

Change all `MiniTest` to `Minitest`
Terminate the backend ourselves on PG 9.2+

This should make it harder to accidentally break this test.
Restore functioning #active? for PostgreSQL

This reverts the substantive portion of 34c7e73.
Test checking #active? while a query is running

This is the behaviour that 34c7e73 was seeking to allow.
Synchronize all access to the underlying PG connection

It's not thread-safe, and we need to safely issue queries from both the
owning thread and the reaper.
No need for #active? to wait around for the lock

If some other thread is using the connection right now, it's clearly
active.

This way, the reaper won't pointlessly get stuck waiting on some
long-running query to complete, just so it can confirm the connection's
still valid.