Add jUnit XML style report generation to start_test.

jUnit XML style reports are a common way of passing tests results along to
other services (e.g. jenkins can parse the results). Add support generating
jUnit reports in `start_test`.

Add new script, util/test/convert_start_test_log_to_junit_xml.py, which parses
the contents of a start_test log and writes out the jUnit XML report. The goal
is to support jenkins, so only elements supported by jenkins junit report
parsing are included.

Update start_test and nightly to support a --junit-xml flag. When provided,
start_test will call the convert_start_test_log_to_junit_xml.py script after
running all the tests. Also add --junit-xml-file flag to start_test. When set,
the jUnit report will be written to the file provided.