Test all dangerous functions and fix check of os.system

[albertvillanova]

Test all dangerous functions and fix check of dangerous function os.system:

• test all dangerous functions, so we avoid issues like:
  • Fix typo in list of dangerous functions #909
• fix check of dangerous function os.system:
  • this issue was found thanks to the test above
  • the module name of this function is posix instead of os

In [1]: import os

In [2]: os.system.__module__
Out[2]: 'posix'