sanitycheck: Fix xml testsuite attribute skipped

In the junit output the attribute containing the number of skipped tests
must be named "skipped" instead of "skip".
See e.g. https://github.com/junit-team/junit5/blob/main/platform-tests/


src/test/resources/jenkins-junit.xsd#L95

Signed-off-by: Christian Taedcke <christian.taedcke@lemonbeat.com>