Testing framework: Test.transfer_* returns gas consumption

goal

Test.transfer_* functions in the testing framework now returns the gas consumed (as a nat).

problems

Contract execution is not the only contributor to the total consumption and I don't think there is a clean way to link each sub-cost to its source. So far this is what I noticed:

• "silent" caching (e.g. typecheking the contrat happens at first execution and not at origination)

• constant cost coming from the operation manager (e.g. a transfer operation has a rather big cost)

• has a changelog entry