Fix inability of rpm-ostree to install multiple packages

[happz]

That's 3 lines removed. The rest of 800+ lines of the patch consist of various improvements to related tests:

• new unit test, uses package managers directly to install multiple packages,
• prepare/install test refactored and extended to cover more distros when testing the basic operations, through parametrization,
• added new custom images to make tests cover "ostree" scenario
• plenty of TODO comments that should be resolved later.

Pull Request Checklist

• [x] implement the feature
• [x] extend the test coverage

[lukaszachy]

So this PR now depends on https://github.com/teemtee/tmt/pull/2828, right?

[happz]

So this PR now depends on #2828, right?

Correct, and adds Alpine among the container images to test prepare/install against.