Fix testcase class results timings on build jenkins

As reported by , the Duration for testcase class results is being reported incorrectly on Jenkins.

To reproduce:

• look at the test results for an edx-platform-python-unittests-master build. e.g. https://build.testeng.edx.org/view/edx-platform-master-tests/job/edx-platform-python-unittests-master/7999/testReport/

• Now drill into some results, eg. pavelib.paver_tests.test_assets

• BUG: the durations of all the classes listed is the same, and incorrect. In this case it says 12 minutes.

• You can see that the test results file from which the timings are derived is correct. In this case it's in artifact/reports/pavelib/paver_tests/shard_1/nosetests.xml, whose contents include:

It looks like this is a bug in the JUnit plugin for jenkins, and was introduced in version junit-1.12 (April 2016).

Further info:

• Note that we recently (Nov 7th, about a month ago) upgraded the version of this plugin from 1.3 to 1.21. See https://github.com/edx/configuration/pull/4198/files

• This PR introduced the bug, seemingly because the author assumed that a testcase suite would only have a single testcase class in it. https://github.com/jenkinsci/junit-plugin/pull/35

• This (open) PR purports to fix the issue: https://github.com/jenkinsci/junit-plugin/pull/66

A few possible options:

• Revert the plugin upgrade

• Champion the fix getting merged in

• Merge the fix into a fork (not sure what version is the highest it will merge cleanly into) and run with that