fixes to tests for py2/3 syntax compatibility

src/python/grpcio/tests/_result.py

src/python/grpcio/tests/_runner.py

src/python/grpcio/tests/interop/methods.py

...hon/grpcio/tests/protoc_plugin/beta_python_plugin_test.py

...thon/grpcio/tests/unit/_adapter/_intermediary_low_test.py

...n/grpcio/tests/unit/framework/interfaces/base/_control.py

...rk/interfaces/face/_blocking_invocation_inline_service.py

...ces/face/_future_invocation_asynchronous_event_service.py