[Security Solution] Rules don't always write a status when finished executing

[marshallmain]

Kibana version: 7.16.0

In the current detection engine implementation, warning statuses are written by rules near the beginning of rule execution. At the end of rule execution, a success status is written only if no warning or error statuses were written during rule execution. This makes it difficult to determine when a rule is finished executing if it writes a warning status. It's especially difficult for integration tests that also expect the rule execution not to write any alerts because we don't have a status document or alert documents, so there's nothing to tell us when rule execution is finished. Thus it's hard to tell if, when we check to see if 0 alerts were written, the rule executed correctly and wrote no alerts or the rule simply hasn't finished yet.

[elasticmachine]

Pinging @elastic/security-detections-response (Team:Detections and Resp)

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)