Provide guidance on running tests

[timvandenhof]

The test project contains many tests, which don't run after checking out the repository.

As these are complete system integration tests which include the communication with the Buckaroo SDK and opening browsers and what not, please provide guidance on how to set up the environment to run te test so other might be able to contribute to this SDK. Same for TestSettings.cs which is intentionally left outside the repository as it might contain sensitive data. We have better options for that these days, so we might be able to improve that.

To improve this I would like to suggest:

• Adding a contributing readme file
• A description on how to run tests and/or set up environment
• Improve credential management for running tests, by using something like UserSecrets
• Improve test scenarios with asserts in stead of having to manually check variables after running a test (which case an IDE message as well: IDE0059 Unnecessary assignment of a value to '{variable}'