testlib: Inconsistency in xml and lightxml logging

In testlib, there are three possible results for a test function or data row: pass, fail and skip. For xml and lightxml output formats, testlib reports passes and fails in an <Incident> tag, but reports skips in a <Message> tag. XFAILs are also reported in an <Incident> tag.

There is no formal definition of these output formats, but my understanding is that the <Incident> tag is for test results (i.e. pass, fail or skip) and the <Message> tag is for other test output that is not reporting a test result (e.g. debug/warning/info messages, XFAILs).