Override test attribute from plan

[lukaszachy]

New option for discover -h fmf to adjust discovered tests

I went with 'adjust-tests' keyword proposed in https://github.com/teemtee/tmt/discussions/2430#discussioncomment-7870532

Pull Request Checklist

• [x] implement the feature
• [x] write the documentation
• [x] extend the test coverage
• [x] update the specification
• [x] adjust plugin docstring
• [x] modify the json schema
• [ ] mention the version
• [ ] include a release note