Add standardized skipped status to junit xml output

With logIncompleteSkipped=false skipped tests are not included in the JUnit XML output of PHPUNit . The JUnit format supports this case via the "skipped" tag - see http://llg.cubic.org/docs/junit/

This change appends the "skipped" tag to the JUnit XML output. We just tested it with Jenkins and it shows up correctly as skipped.

It may be a breaking change for tools processing JUnit XML reports - does it make sense to beta test this?