Add GitHub action

[SplittyDev]

General

This PR adds a Github Workflow which triggers on push and pull_request.
The workflow is modeled very closely after the existing CircleCI workflow.

Motivation

Merging this will improve the general QA workflow a lot, since pull requests from forks will be testable without creating a separate branch.

Limits

There is no action limit for public repositories.

[SplittyDev]

Seems like this is working now, however a very small amount of tests takes longer than a second and even rarely fails with a 2s timeout. Not sure why this happens, but I guess in that case we can always just re-run the test.

For now I updated all tests to use a timeout of 2s, which should also help with sporadically failing CircleCI tests.