Setup liveness heuristics inside the BugFindingEngine

[pdeligia]

Right now, liveness heuristics are setup at the PSharpTester level, we should instead setup them (as well as any other heuristics) inside the TestingServices library. The reason behind this is that someone who uses the tester, might set e.g. max-steps (which automatically sets fair/unfair steps for liveness), but when running a test programmatically from e.g. Xunit or MSTest, they have to know about these heuristics and set them by hand. This is confusing and error prone, and we should fix it.