Add minimal tests for recovery conflict handling.

Add minimal tests for recovery conflict handling.

Previously none of our tests triggered recovery conflicts. The test is
primarily motivated by needing tests for recovery conflict stats for shared
memory based pgstats. But it's also a decent start for recovery conflict
handling in general.

The only type of recovery conflict not tested yet are rcovery deadlock
conflicts.

By configuring log_recovery_conflict_waits the test adds some very minimal
testing for that path as well.

Author: Melanie Plageman <[email protected]>
Author: Andres Freund <[email protected]>
Discussion: https://postgr.es/m/20220303021600[email protected]