GitLab CI: GCC CPU and Sycl CPU do not run ctest

[SimeonEhrig]

At the moment, the CI does not run ctest on the CPU runner if CPU backends is used on GitLab CI. This is a left over from the beginning of using GitLab CI with the GPU tests. This needs to be fixed.

[j-stephan]

For SYCL this is intended since there are known failures.

[fwyzard]

Last week we discussed the possibility of marking the known failures as such, but still running at least the other ones.

[SimeonEhrig]

I start to implement the mechanism to run GCC CPU tests. Adding sycl afterwards should be easy and fast.