Fix run test scripts

[flxst]

What does this PR do?

This PR is a follow-up to #241 and fixes a few minor things:

• validity of 3rd input argument (--cov / --no-cov) in run_distributed_tests.sh is checked
• run_distributed_tests.sh is called with --no-cov from tests/tests.py
• unused 3rd input argument in run_all_tests.sh is removed

General Changes

• none

Breaking Changes

• none

Checklist before submitting final PR

• My PR is minimal and addresses one issue in isolation
• I have merged the latest version of the target branch into this feature branch
• I have reviewed my own code w.r.t. correct implementation, missing type hints, proper documentation, etc.
• I have run a sample config for model training
• I have checked that all tests run through (python tests/tests.py)
• I have updated the internal changelog (CHANGELOG_DEV.md)

[le1nux]

Thanks for improving this! LGTM :-)