[SUREFIRE-996] <system-out> and <system-err> content of junit xml report are not escaped

Testcase by Julien Vermillard. Fix by me. Testcase extended by me.
Do not enforce a new lifecycle in ReportOnly Mojos to speed up site build.
Let SurefireReportOnlyMojo be a child of AbstractSurefireReportMojo to avoid execution of phase validate.