Write logs in junit xml

Added the ability to write logs in junit xml (if logcapture plugin is enabled).
Logs is written into the `<system-err>` section of testcase. If there are no logs, the `<system-err>` section will be empty (but it always exists).

I need a suggestion how to test it (should I enable 2 plugins? of just check that `<system-err>` exists?).

Related issue #56