Extend interoperability tests to use async provider (#1312)

The interoperability tests exercise a well-defined sequence of requests
for all implementations of gRPC. We already had interop tests for the
ELG-based client and provider. This pull request extends the tests to
exercise an async-await–based provider.

Signed-off-by: Si Beaumont <beaumont@apple.com>

Author: simonjbeaumont