Add support for cobertura

[elernonelma]

Since release https://github.com/sebastianbergmann/php-code-coverage/blob/master/ChangeLog.md#920---2020-10-02 phpunit is able to generate cobertura report: phpunit --coverage-cobertura=cobertura.xml

We use it in our gitlab factory and works fine - do you have any plan to work on it? I may have some time to look at an implementation but not until maybe mid december.

[Jean85]

Implementation should be pretty straightforward, since we reuse PHPUnit components to generate coverage. PR appreciated!