TST : tweak test_add_collection

This is needed because this test tries to add artists from one axes to another which is not really supported. Not 100% that this is fully equivalent to the current test. There is a call to `add_collection` in scatter, but the test might be defeated by some of the other logic in scatter.
matplotlib · efiring · Dec 13, 2014 · Nov 23, 2014 · Nov 24, 2014 · Nov 27, 2014
commit fa87fa74f4ac01c1979b58a8927a0fa01ac289d3
diff --git a/lib/matplotlib/tests/test_collections.py b/lib/matplotlib/tests/test_collections.py
@@ -407,14 +407,12 @@ def test_null_collection_datalim():
 def test_add_collection():
    # Test if data limits are unchanged by adding an empty collection.
    # Github issue #1490, pull #1497.
-    ax = plt.axes()
    plt.figure()
-    ax2 = plt.axes()
-    coll = ax2.scatter([0, 1], [0, 1])
+    ax = plt.axes()
+    coll = ax.scatter([0, 1], [0, 1])
    ax.add_collection(coll)
    bounds = ax.dataLim.bounds
-    coll = ax2.scatter([], [])
-    ax.add_collection(coll)
+    coll = ax.scatter([], [])
    assert_equal(ax.dataLim.bounds, bounds)