Add unit testing

[rsubtil]

Now that addon is becoming increasingly more complex with performance and multi-threading related fixes, and seeing it being used more and more on professional projects, unit testing should be implemented thoroughly on the project to minimize future changes that can have significant impacts on unrelated code.

The idea is to use an existing unit testing framework (GUT or gdUnit4), and create tests of two types:

• Functional tests, covering basic and advanced functionality of the addon, including very specialized tests for complex scenarios reported in the past. The objective is to not allow any regression to slip by.
• Performance tests, covering performance related scenarios that serve as a "canary" to alert for performance regressions. While these won't be as strict as functional tests in implementing new features, it should still be regarded carefully to avoid death-by-a-thousand-cuts scenarios.