testeth: ignore test cases with no suitable transaction

[s1na]

I got the error:

error: in "GeneralStateTests/stShift": Transaction not found! (Test: 'shiftCombinations', Network: Byzantium, dataInd: -1, gasInd: -1, valInd: -1)

when running testeth with the flag --singlenet Byzantium. Looking at the test case it seems it has no transaction for Byzantium. I expected testeth to ignore such a test case.