Improved Lint and Testing

[spacecheese]

• Add testbench linting
• Implement asynchronous Adapter.discover_devices
• Merge ServiceInterface basic export implementation (some extra components now have globally consistent default export paths)
• Refactor background_service, background_bus and background_advert fixtures
• Modify and extend tests for better coverage
• Implement error wrapping mechanism with unique classes per dbus error type
• Remove test with trial service/ advert registration from unit (these will be part of more accurate loopback testing in future)

Fixes #54
Fixes #58
Fixes #55
Fixes #32