Update InitialSyncTest unit test to avoid hardcoding FCV constants

In InitialSyncerReturnsParseErrorWhenFCVFetcherReturnsNoVersion, we hardcode an FCV 4.4 constant. As a result, after upgrading to 5.0 LTS, this will break. We should update the unit test to use a GenericFCV constant or find some alternative way of testing the behavior.