Create automated tests

[Dovyski]

All existing tests regarding cvui only check if it compiles using different versions of OpenCV. This is not enough. Tests should at least inform if a program using cvui compiles and runs successfully. PR 46 adds support for a headless version of cvui, which is perfect for creating tests.

Following the same logic of the examples, tests should be under the folder tests in the root of the project. They should not depend on complex dependencies, e.g. a header only lib for testing would be ideal.

[Dovyski]

Here is a nice lib for the job: https://github.com/silentbicycle/greatest