Various improvements to testhelpers.
* WaitForActiveNodeAndPerfStandbys is used to make sure a cluster is fully ready, i.e. both its active node and perf standbys are in a good state.
* WaitForReplicationStatus is like WaitForReplicationState but uses the API, part of a general
effort to move us away from interacting with Core directly in these tests.
* WaitForPerfReplicationWorking is similar to some code that exists in ent already: it writes to the primary and waits to see that appear on the secondary.
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