Rework testing

[spacecheese]

Add tests for untested functionality

• [ ] Advert release callback
• [ ] Remainder of util functions
• [ ] Documentation example code
• [ ] (Better) Async characteristic/ descriptor tests
• [ ] Refactor existing tests to use a better mocking design

Add additional bluez integration tests

• [x] Implement testing images (see bluez_images)
• [ ] Characteristic
• [ ] Descriptor
• [ ] Service
• [ ] Agent
• [ ] Advertising

Other

• [ ] Add code coverage checking