As the title says. We have at least 2 scenarios when this would be needed:
1) We want to build qtwayland before qtbase tests are run, so that qtbase tests are run on top of the new qtwayland.
2) If we have a compilation error in say qtdeclarative and you are trying to fix that. You don't need to run tests for qtbase, before you see that you actually fixed the compilation error. So this could save some resources in some rare cases.