RollbackTest shouldn't run serverStatus against node while it may be in rollback

The RollbackTest fixture runs serverStatus against a node to see whether it supports majority reads or not. At this point in the test, though, that node may be in the middle of rollback which will cause the command to fail. We should run this serverStatus command at an earlier point in the test when we are sure the node will be in a steady state and not in rollback.