In SERVER-50154 we added a Mock Network Fixture to make testing network messages less of a headache. As part of this ticket we added a test using this mock to both initial_syncer_test and replication_coordinator_impl_elect_v1_test

This ticket will server as a spike to learn how to use the Mock Network Fixture to be able to use it in more complicated POCs. We should time box this ticket to around a week, the goal is not to convert a specific number of tests over but to get some more tests added in each file while learning how to use the fixture.