Explicitly check overwrite works for existing file in tests

Author: Luke Shaw

tests/ndarray/test_concatenate.py

@@ -93,3 +93,9 @@ def test_stack(shape, dtype, axis):
 
     newres = blosc2.open("localfile.b2nd", mode="r")
     np.testing.assert_almost_equal(newres[:], nparray)
+
+    # Test overwriting existing file
+    result = blosc2.stack(
+        [ndarr1, ndarr2, ndarr3], axis=axis, cparams=cparams, urlpath="localfile.b2nd", mode="w"
+    )
+    np.testing.assert_almost_equal(result[:], nparray)