Comprehensive test suite for libindigo clients

[chrsoo]

Description

Add a libindigo-test crate that contains functionality for running unit and integration tests by libindigo-rs and libindigo-ffi users.

Goals

• Rust only mock server implementation for fast integration tests.
• Developer friendly tests for async code.
• Reuse of libindigo-test crate code.
• Good documentation showing how to use the crate for testing.

Task List

TODO: break down this Epic into manageable sub tasks and add them to the list below.

• Implement Lightweight Mock INDIGO Server #43
• Runnable example tests using the framework

Additional Notes

[!NOTE] some thought must go into avoiding circular dependencies between the main this crate and other crates to meet the last goal.

[!NOTE] The test crate should provide support for running a real INDIGO server running devices implemented in libindigo-rs.