Users of the high-level API should not care whether we're using AF_INET
or AF_INET6 sockets internally.

This fixes intermittent connection issues on 32-bit Linux. Also related
to issues #2557 and #2600.

This introduces grpc_test.test_common for gRPC-specific test code and
fixes issue 2570.

Also updates the version of grpcio_test. Now that grpcio and its tests
are in different project packages, the version numbers should be kept in
sync.

This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.