Make TestReportFormat, specifically the `junit` format selectable

[haeser]

Context

Circleci and probably other tools as well need the Junit xml format in order to properly parse test runs. Support for both the sonar generic test execution format and the Junit XML format were already implemented 👏🏼

This PR simply adds the option to select the junit test report formatter from the cli tool. Default is still the sonar generic test execution format, the change is non breaking.

Summary of changes

• Add new junit output format
• Improve tests by checking for root element name
• Update docs
• Bump version

[a7ex]

Hi Florian, thanks for your contribution. It totally slipped my attention, that, what you were after was implemented, but inaccessible due to the missing command line argument, which you now added. Great! Also the improvement to the very sparse (as usual ;-) unit tests is very welcome.