Currently gitlab-ci doesn't allow variables in `exists` rules, which
means the condition below can't be used. This is fixed by moving the
condition into the script and just exit successful if the tests don't
exist.

Further it adds a requirement for the chart-package job to wait for the
chart-helm-unittest job(s) before, this should help to prevent the
release of broken charts.

Finally we namespace the junit files generated during the tests, since
they would overwrite each other, if multiple helm charts would run the
CI jobs at the same time.

References:
https://docs.gitlab.com/ee/ci/yaml/#needs
https://docs.gitlab.com/ee/ci/yaml/#rulesexists