Clean up convert-trs-to-junit.py invocations

Clean up convert-trs-to-junit.py invocations

  - Use absolute paths when invoking the convert-trs-to-junit.py script
    so that it also works correctly for out-of-tree and tarball-based
    test jobs.

  - Quote the variables used in convert-trs-to-junit.py invocations to
    future-proof the code.

  - Use "&&" instead of ";" in shell pipelines invoking the
    convert-trs-to-junit.py script in order to prevent "source" errors
    from being silently ignored.

  - Ensure convert-trs-to-junit.py is invoked from the correct directory
    for out-of-tree and tarball-based unit test jobs by adding
    appropriate "cd" invocations.

  - Ensure the convert-trs-to-junit.py invocations are always the last
    step in each 'after_script', in order to run that script from the
    correct directory for out-of-tree and tarball-based system test jobs
    and to ensure that any potential errors in that script do not
    prevent more important steps in the 'after_script' from being
    executed.