Add integration tests for core addins

[pascalberger]

We should add integration tests for core addins (Cake.Issues, Cake.Issues.PullRequests and Cake.Issues.Reporting), which test the addin on different operating systems and with different runners (like e.g. done here https://github.com/cake-contrib/Cake.Issues.Reporting.Generic/#build--test-status).

[pascalberger]

This should be easier once code base has been merged into this repo (#391), as some of the integration tests which already exists for some addins will cover the functionality of the core addins.