tests: provide a simple way to extract the tests data from run

If the RUN_OUTPUT_FILE environment variable is set, "run" will output
to the file indicated by that snippets of XML with the results and
outputs of the tests run.

Together with the run-xml-to-junit.sh (and its associated
run-xml-to-junit.xsl style sheet) it is possible to convert that output
file to a jUnit-like XML file, which could be used in CI systems.
tests: skip the output of test-virt-rescue.pl from RUN_OUTPUT_FILE

The output of test-virt-rescue.pl contains non-ASCII chars which give
troubles when being XSLT-transformed, so just avoid its output to leave
the rest of the resulting XML well-formed.