A single gitlab-runner can run multiple test stages
concurrently on the same hardware. When we have 3
jobs asking for 100% of the cpu's we get a 300%
reduction in performance caused by task switching
Problem was that CMake 3.12 was the CMake version used in the
build image, and test image was using 3.13. This was a problem
as the install test invocation aren't backwards convertible and
therefore failed.