Cast iterables into iterators for stream based compression, empty
message, and metadata tests.

Moves all tests into a separate package. This does not change existing
supported means of running tests (e.g. through run_tests.py).

This impacts the following APIs:
  Metadata: Key is always a str, Value is bytes for binary metadata,
    str otherwise
  Call Details: str type
  gRPC method: str type
  hostname/target: str type