tests/robustness: check for compaction before prevKV validation

We can check for the condition that Kubernetes checks for, i.e.
prevKV can be nil iff the event is not a create a event, only if
we know whether compaction has occured or not. If compaction has
occured, prevKV can be nil and that is completely valid.

This commit checks if compaction took place during the test run
by querying the /metrics endpoint. Based on if compaction occured,
we now check the Kubernetes condition in the prevKV robustness test.

Signed-off-by: Madhav Jivrajani <madhav.jiv@gmail.com>