Tests: Enable RunCMake.CPack_External test more reliably

In commit 2ef966bc77 (Testing: Add test for CPack External generator,
2018-06-11, v3.13.0-rc1~361^2~2) 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).
1 file changed