Add unit tests for fault injection in Service Disruptor

[pablochacin]

Initially, the ServiceDisruptor was implemented as a wrapper of a PodDisruptor (created using the service's selector to create the PodSelector). The ServiceDisruptor's fault injection methods simply passed the arguments down to the respective PodDisruptor methods and therefore did not have any unit test.

The current implementation has diverged and now the ServiceDisruptor has its own logic for fault injection that should be tested to detect faults such as #260.

[pablochacin]

Blocked by #301