Add NUnit XML support to test babysitter, also document the communication protocol...

Add NUnit XML support to test babysitter, also document the communication protocol better

For test harnesses which don't support the full babysitter protocol,
the test harness can now report back paths to NUnit-format XML files
containing test results, or the paths to those XML files can be given
by whoever invoked the babysitter script. Failures listed in the XMLs
are included in the babysitter log.

test-runner.cs has been modified to report its XMLs, so we should now
see mono/tests failures in the babysitter log.