(#123) enabled building on linux

[nils-a]

and added CI-Builds for linux and mac.

This is mostly changes to the unit test which were expecting a very windows-like setup.

Description

Additionally, I have added a reference to Cake.Testing and removed some custom fakes that are now available in Cake.Testing.

Related Issue

fixes #123

Motivation and Context

123

How Has This Been Tested?

Since it's mostly changes to the tests itself, they are inherently tested. I ran the tests locally on Windows 10 and Ubuntu 18.04 as well as Ubuntu 20.04.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change) Changes to the build-process.

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[gep13]

@nils-a did you want to rebase this PR on the head of the develop branch, and then we can look to get this merged in. Thanks