Add unit tests

[limbonaut]

As discussed in #5, we need to decide how to approach testing.

There are at least two testing frameworks for GDScript. Tests can be implemented in GDScript within a Godot project using a testing framework addon. Such tests could be placed in the demo project or in a separate project dedicated to testing. The main advantage of this approach is that it executes code via GDScript - just like users would - and provides API usage examples. This setup makes it easier to verify if we're breaking the public API.

Another option is to use one of the C++ testing frameworks. Godot Engine already uses the C++ doctest framework, and any Godot module can add its own tests. If we add an option to build Sentry SDK as a Godot module, we could also add tests using this framework. Example: https://github.com/limbonaut/limboai/tree/master/tests. The advantages of this approach include no extra dependencies, and the ability to test internal things that are not part of the public API. However, sentry-godot can't currently be compiled as an engine module, and it would require some work to port. Such tests are compiled with the engine (using a specific build option) and can be executed via a CLI option, like so: godot --test --tc="*[SOME_TEST_GROUP]*"

We need #16 for CI integration.