Change the default formatting of YAML emitted by tmt

[happz]

By setting mapping, sequence, and offset formatting hints for the YAML dumper, the produced YAML incorrectly handles multiline strings starting with an empty line, and export of collection items under tmt * export.

After some experimenting, there seem to be no silver bullet setting ticking all boxes WRT YAML formatting, but resorting to setting just sequence=2 seems to produce a reasonably formatted YAML.

Note that this does not affect YAML - or fmf - accepted by tmt, only YAML/fmf files written by tmt.

Fixes https://github.com/teemtee/tmt/issues/3432

Pull Request Checklist

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

[happz]

Blocked by https://github.com/teemtee/tmt/pull/4334