No unit tests for query runner status pathway

[dmutreja]

There are no unit tests for the server implementation of Status(..) RunnerStatus rpc in on the runner gRPC interface.

In the current implementation requested RunnerStatus can be built from a combination of
a) default counters
b) optional status container image
c) optional pluggable health checker function

Propose refactoring type status_tracker out from https://github.com/fnproject/fn/blob/master/api/agent/pure_runner.go to a new file and adding unit tests to cover the supported cases.

Further, status_tracker type is becoming somewhat un-weildy and needs to be refactored. The unit tests would help maintain correctness if/when we chose to do so.