Cleanup GTests for simple_switch_grpc

[antoninbas]

https://github.com/p4lang/behavioral-model/tree/master/targets/simple_switch_grpc/tests

We currently create one switch instance which is used for all the tests. This is suboptimal as it means some state is preserved across tests (e.g. port operational status) and a failing test may cause other unrelated tests to fail as well.