Add simple unit test
Here is a simple example of implementing unit tests into a quest using the latest CI pipleines from @hhromic.
This unit test verifies the fix from https://gitlab.com/solarus-games/solarus-free-resource-pack/issues/16 (which was already merged a while back)
Special Note!
Before merging this MR, the environment variable SKIP_PACKAGING
found under Settings > CI / CD should be set to any value so that the .solarus package is not generated (which does not make sense for the free-resource pack). See image below.