stepUpNewPrimaryOnShard attempts to step up a random secondary, then if step up command is successful, asserts that the current primary is the one that we just stepped up.

However, as seen in BF-37901, it's rarely possible for a failover to occur between stepping up the secondary successfully and verifying the current primary.

In this case, the test fixture should retry if the actual primary changed back to the original primary, otherwise accept that one of the other secondaries ended up stepping up.