Improve TestEnv ; fix xdist detection, and fix EnvironmentError

- Detect the presence of xdist and display a message accordingly, before calling pytest with or without "-n cpu_count".
- Improve TestEnv, mostly comments and pep8 validation, but also catch "TypeError" exception when we call unittest.TestCase.assertAlmostEqual() on String for instance.
- EnvironmentError exception was badly converted to string when argcount > 3, the issue is that BaseException store the argument list as a ... list. It should maybe be stored as a tuple (python semantic). The fix is then maybe not the right way to go?