Viewer: Test running of the viewer in the CI

[PLukas2018]

It would be good to try to run the result viewer in the CI to check that it works correctly. The run should be added here: https://github.com/diffkemp/diffkemp/blob/3d6ca2a272a74f11cea87e8105295a8602e79a16/.github/workflows/builds.yml#L72-L78

Both the development version and the release version of the viewer should be run.

To be able to run the viewer, the diffkemp compare command must find a semantic difference. So, this would probably require creating a new make-based testing project, which would contain semantic differences against the current one.