[OE-core] Enable xml output in OeTest class for report consumers

[Benjamin Esquivel]

You can take a look at the bug 9682[1] where tests and sample logs are attached.
This patch assumes the xmlrunner[2] module in the system where oe-selftest will
run. if the xmlrunner is not there, it will default to unittest.

As it is now, it will change partially the text output format of the command
line but a bug will be filed for fixing that up. Reason being that is better
to fix it at the xmlrunner than recreating the functions to fix it. See the
summary of changes at the bug's attachments.

[1] https://bugzilla.yoctoproject.org/show_bug.cgi?id=9682
[2] https://github.com/xmlrunner/unittest-xml-reporting