tests: Replace usage of tostring() with tobytes()
tests: Replace usage of tostring() with tobytes()

tostring() is deprecated as of python 3.7 and results in stderr output
causing tests to fail

DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin