filenames in junitxml output

[pytestbot]

Originally reported by: Ronny Pfannschmidt (BitBucket: RonnyPfannschmidt, GitHub: RonnyPfannschmidt)

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would help with after the fact timing analysis without needing a file tree

could be either a name-spaced attribute on the testcase elements,
or a element within them

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• Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/145

[The-Compiler]

This should be fixed via #809