Add vectorarray cow test (#1419)

This adds a test for copy on write of VectorArrays and increases the `max_examples` count to 20 in `dev` and `debug` sessions.

I stumbled upon broken COW functionality when binding a vector from an external PDE solver, and this was not catched by our tests.

src/pymortests/vectorarray.py

@@ -266,6 +266,29 @@ def test_copy(vectors_and_indices):
             pass
 
 
+@pyst.given_vector_arrays(index_strategy=pyst.valid_indices)
+def test_COW(vectors_and_indices):
+    v, ind = vectors_and_indices
+    for deep in (True, False):
+        if ind is None:
+            c = v.copy(deep)
+            assert len(c) == len(v)
+        else:
+            c = v[ind].copy(deep)
+            assert len(c) == v.len_ind(ind)
+        assert c.space == v.space
+        if len(c) > 0 and not np.all(c.norm() == 0):
+            c *= 2
+            if ind is None:
+                assert not np.all(almost_equal(c, v))
+            else:
+                assert not np.all(almost_equal(c, v[ind]))
+            try:
+                assert np.allclose(c.to_numpy(), 2*indexed(v.to_numpy(), ind))
+            except NotImplementedError:
+                pass
+
+
 @pyst.given_vector_arrays()
 def test_copy_repeated_index(vector_array):
     v = vector_array