Update unit tests
We need to do the following for gracedb-client's unit tests:
- Add unit tests using Mock: the current tests are really integration tests since they depend on the server. It is important to maintain the integration tests, too.
- Make the unit tests "discoverable" so they can be run as part of a packaging procedure. The integration tests should not be run at that point.
- Add unit and integration tests for the new superevent functionalities.
Edited by Tanner Prestegard