Add test_runner support to the test command

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Originally reported by: ldore (Bitbucket: ldore, GitHub: ldore)

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For our nightly builds, I need to use a different test runner, that outputs junit compatible xml files.

Back in 2008, Klaus Zimmermann submitted a patch for this: Add test_runner support to setup.
(This only adds support for the test runner, NOT for junit output. You'll have to use XmlTestRunner for junit output.)

The patch was deemed acceptable, but never made it to the trunk.

I've been using this patch for a few years, with good results.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/176

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Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Apply patch based on patch in 2008 by Klaus Zimmerman. Fixes #176.