Minor replication test cleanup tasks

[roji]

This tracks various minor test tasks scoped out from #3281 in the initial replication implementation.

• Possible cleanup of tests to remove genericity (Implement the PostgreSQL Streaming Replication Protocol #3281 (comment))
• Better identifier management in tests (Implement the PostgreSQL Streaming Replication Protocol #3281 (comment))
• Remove useless CancellationToken.None in tests (Implement the PostgreSQL Streaming Replication Protocol #3281 (comment))
• DRY test teardown logic (Implement the PostgreSQL Streaming Replication Protocol #3281 (comment))

/cc @Brar