Add a new unit test for clone env leakage.

[psigen]

After the discovery of #286, we should add a unit test that verifies that cloning and planning operations do not leak OpenRAVE environments. This would be something to the effect of:

import openravepy

robot.PlanToConfiguration(robot.GetActiveDOFValues(), execute=True)
initial_num_envs = len(openravepy.RaveGetEnvironments())
for i in range(10):
    robot.PlanToConfiguration(robot.GetActiveDOFValues(), execute=True)
assertEqual(initial_num_envs, openravepy.RaveGetEnvironments())

I am not sure exactly how to do this test in prpy directly (without herbpy or similar).

[mkoval]

It may be best to put this code in each *py package. It's possible for a robot to introduce circular dependencies in its CloneBindings functions, so that is the most reliably place to do the test.