Plotter test fails when executed more than once

This itself is not a problem, but (probably) the same problem shows up when running nosetests with more than one process. Also, this function should seed the RNG.

$ python -c 'import kwant.tests.test_plotter as tp; tp.test_mask_interpolate(); print "2nd"; tp.test_mask_interpolate()'
2nd
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "kwant/tests/test_plotter.py", line 169, in test_mask_interpolate
    assert len(w) == 1
AssertionError

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