generate-tests aware tmdir breaks if test id contains the path separator

Originally reported by: **Ronny Pfannschmidt (BitBucket: [RonnyPfannschmidt](http://bitbucket.org/RonnyPfannschmidt), GitHub: [RonnyPfannschmidt](http://github.com/RonnyPfannschmidt))**

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happens since the test id is used for making the path
i cant think of a good solution atm

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- Bitbucket: https://bitbucket.org/pytest-dev/pytest/issue/4
