Refactor/cleanup TestReplicationScenario

TestReplicationScenarios could use a bit of cleanup, based on comments from reviews:

a) Separate "setup" phase of each test, so that we don't run unnecessary verifications which aren't testing replication itself, but are verifying that the env is set up correctly to then test replication. This can be flag-gated so as to allow it to be turned on at test-dev time, and off during build/commit unit test time.

b) Better comments inside the tests for what is being set up / tested.