Handle `--log-junit` option

[Jean85]

The --log-junit would be pretty useful to report tests to various systems, including the GitLab CI Unit Test Report: https://docs.gitlab.com/ee/ci/testing/unit_test_reports.html

This option is not feasible as a pass-through one, because it would produce multiple files, and also because there's no way to produce different files with different names for each executed test class.

This feature would mirror the coverage one, with the hijacking of the native PHPUnit option, and the merging of the resulting files. The merge would have to be written as a feature, but would be pretty straightforward, see https://github.com/drazisil/junit-merge/blob/master/lib/index.js