Add unit tests for the network fault injector #2
Description
As the NetworkFaultInjector grows in complexity (I'm adding host <-> container fault injection, for example), we should add standalone tests of the fault injector itself. These tests should only depend on the testing framework itself and not on Spark. This will help us to diagnose cross-platform portability issues with the fault injector.
The tests that currently use the fault injector use assertions to check that connection were actually severed (for example, by checking that workers disassociate from the master), so this should guard against tests that pass by accident due to a broken fault-injector. We still need tests for the injector itself in order to help determine whether test failures are legitimate or are due to problems in the framework.