fix(test): new scripts/run-tests

[razvan]

This adds a new helper for running tests called script/run-tests.

This script takes the same arguments as the old scripts/run_tests.sh but improves
it's behavior in a few ways.

It enables users to run custom CI tests in Jenkins without having to patch
the release.yaml file in pull requests.

This is possible given a new argument --operator <name>=<version>. When provided,
the operator version specified om the command line is installed instead of
the version present in the release.yaml file.

See ./script/run-tests --help for more information.

It also solves the problem that stackablectl doesn't exit with a non-zero code
on failure and therefor the CI pipeline may continue to run with a broken
operator installation.

[sbernauer]

Did not review the code properly (as @soenkeliebau assigned himself), but the functionality looks good to me, thanks!