Add rerun capability to pytest test failures

[johnmhoran]

I've noticed with a recent PR (#2958) that one test in particular (FAILED tests/scancode/test_cli.py::test_scan_does_scan_php_composer) seems to fail with near-regularity, yet often will subsequently pass after being manually rerun.

As @pombredanne pointed out when we were discussing how best to handle this behavior, https://github.com/pytest-dev/pytest-rerunfailures may offer an effective method to address these sorts of intermittent failures. Fingers crossed. ;-]

[pombredanne]

Thanks! actually this is more than nice to have as these random CI are a pain as they make automation fail at automating.