ci/cd: make workflow pipeline more robust

[lukpueh]

Description of issue or feature request:
#1946 adds a CD workflow to release build artifacts on PyPI and GH upon successful completion of the CI workflow for a pushed release tag.

Unfortunately, the information about CI that we can access from CD is vague and thus makes the workflow pipeline a bit brittle (see details below).

Current behavior:
CD triggers for each completed run of CI, regardless of the ref that triggered CI, but then skips execution if the CI run was not successful or the ref (branch or tag) that triggered CI does not start with a 'v'. In order to not trigger CD for a branch that starts with 'v', we rely on the configuration of CI, which currently does not trigger on other branches than 'develop'.

Expected behavior:
CD triggers for completed and successful run of CI but only if CI was triggered by a release tag, regardless of CIs trigger conditions.

Alternative solution:
Restructure workflows, so that all test logic from 'ci.yml' is moved to a separate workflow file '_test.yml', that can be included in both the CI workflow, which then only triggers on push to 'develop'-branch, and the CD workflow, which then only triggers on on push to 'v*'-tag.