get rid of side effect in test

kwant/tests/test_plotter.py

@@ -151,7 +151,7 @@ def test_map():
         with warnings.catch_warnings():
             warnings.simplefilter("ignore")
             plotter.map(sys.finalized(), xrange(len(sys.sites())),
-                              file=out)
+                        file=out)
         nose.tools.assert_raises(ValueError, plotter.map, sys,
                                  xrange(len(sys.sites())), file=out)
 
@@ -160,7 +160,6 @@ def test_mask_interpolate():
     # A coordinate array with coordinates of two points almost coinciding.
     coords = np.array([[0, 0], [1e-7, 1e-7], [1, 1], [1, 0]])
 
-    warnings.simplefilter("ignore")
     with warnings.catch_warnings(record=True) as w:
         warnings.simplefilter("always")
         plotter.mask_interpolate(coords, np.ones(len(coords)), a=1)
@@ -168,7 +167,9 @@ def test_mask_interpolate():
         assert issubclass(w[-1].category, RuntimeWarning)
         assert "coinciding" in str(w[-1].message)
 
-    assert_raises(ValueError, plotter.mask_interpolate,
-                  coords, np.ones(len(coords)))
-    assert_raises(ValueError, plotter.mask_interpolate,
-                  coords, np.ones(2 * len(coords)))
+    with warnings.catch_warnings():
+        warnings.simplefilter("ignore")
+        assert_raises(ValueError, plotter.mask_interpolate,
+                      coords, np.ones(len(coords)))
+        assert_raises(ValueError, plotter.mask_interpolate,
+                      coords, np.ones(2 * len(coords)))