AtlasBuildScripts: enable build of CITest package

Enable the build of the tests in the CITest package when the project is built as part of the CI.

@akraszna, initially I thought that we'll modify the CI-cmake-configuration Jenkins job to run build.sh ... -c -a -x -DATLAS_ENABLE_CI_TESTS=TRUE. But then I realized that e.g. our 21.x branches do not support the -x argument and we would have to distinguish branches in the Jenkins job. So this solution seems simpler for the moment. What do you think?

Relates to ATLINFR-4360.