The xunit plugin claims to be compatible with jenkins-ci junit-4.xsd,
but it does not pass on the validation dur to the wrong key name 'skip'.

This patch changes the key name to 'skipped' and adds a unittest to validate
the xunit output against the junit-4.xsd schema.

The junit-4.xsd file is coming from:
https://svn.jenkins-ci.org/trunk/hudson/dtkit/dtkit-format/dtkit-junit-model/src/main/resources/com/thalesgroup/dtkit/junit/model/xsd/junit-4.xsd

Reference: https://trello.com/c/ZdEb0TblSigned-off-by: NAmador Pahim <apahim@redhat.com>