Tests: Enable RunCMake.CPack_External test more reliably
In !2142 (merged) this test was added without defining its get_test_prerequirements
function, so it has been re-using the conditions from the previous test type (STGZ).
In !2142 (merged) this test was added without defining its get_test_prerequirements
function, so it has been re-using the conditions from the previous test type (STGZ).