Auto merge of zcash#3307 - gtank:3187-refactor-checkblock, r=bitcartel

Refactor ContextualCheckBlock tests

This cleans up and condenses our quadratically-scaling transaction format / consensus rule tests. They still technically scale quadratically, but should be much more readable and manageable now - all that remains in the individual test cases is the specific version flag tweaking we're trying to test.

Closes zcash#3187.

Author: zkbot