Fix flakey base setitem test
This test makes a potentially incorrect assertion about the data provided to the test. We already require that `data[0] != data[1]`, so it can be used instead.

This failed in https://dev.azure.com/pandas-dev/pandas/_build/results?buildId=24731&view=logs&j=a67b4c4c-cd2e-5e3c-a361-de73ac9c05f9&t=33d2fdd0-c376-5f94-e6d3-957bdd23a3b8.
Thanks @TomAugspurger 