The new spec uses existing EchoDuplicate service in
grpc.testing.duplicate, which refers grpc.testing.Empty,
and verifies if the plugin generates this correctly as
Grpc::Testing::Empty.

Some __dealloc__ methods were calling Python methods, and some
references were being dropped on the floor instead of threaded through
gRPC core.

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

Also set up environment-related details needed for a smooth Cython
experience: have the test script avoid rebuilding all dependencies if the
virtualenv directory already exists, have the PyPI distribution script
distribute the Cython generated C code rather than the .pyx files.