use nonrandom coordinates in test_mask_interpolate, closes #1

kwant/tests/test_plotter.py

@@ -158,9 +158,7 @@ def test_map():
 
 def test_mask_interpolate():
     # A coordinate array with coordinates of two points almost coinciding.
-    coords = np.random.rand(10, 2)
-    coords[5] *= 1e-8
-    coords[5] += coords[0]
+    coords = np.array([[0, 0], [1e-7, 1e-7], [1, 1], [1, 0]])
 
     warnings.simplefilter("ignore")
     with warnings.catch_warnings(record=True) as w: