Convert JUnit XML from pytest into Automake .trs files

Convert JUnit XML from pytest into Automake .trs files

It's important to parse the JUnit result file rather than relying on the
exit code from pytest, which has a different meaning. Include a .trs test
result for each test case and set an exit code which is most appropriate
as the aggregate result (e.g. it will be set to 77 (SKIP) if there's at
least one test case that was skipped).