Investigate coverage of FAST sync in Acceptance Tests

[RatanRSur]

#384 exposed that some acceptance tests fail on circleci (though not locally on my machine) when --sync-mode=FULL is not set.

The previous default was FULL sync always so I assumed that there must be some acceptance tests that explicitly enabled fast sync in order to test that part of the code. However, I discovered that there isn't even a way to configure the sync mode, and so, all ATs were using FULL sync.

We need to spend some time investigating how/if we want fast sync coverage in the acceptance tests. There are other forms of testing we can rely on for fast sync, unit, integration, ethereum/hive, to name a few. We should just make this decision intentionally and then go forward with whatever we decide.

[RatanRSur]

We only have one test for fast sync that's more involved than a unit test and that's in RunnerTest. We're going to rely more heavily on the hive fast sync tests going forward do there's no action item to write new fast sync test in besu.