[skip changelog] Migrate tests to GH actions

[masci]

This PR migrates the test pipelines running on drone.io and Appveyor to Github actions providing the following improvements:

• Now the test pipeline is (with minor differences) the same for Windows and Linux
• We can converge to using only one CI instead of two different services
• We can easily add OSX to the operating systems matrix where the test pipeline runs

[rsora]

Excellent!