TST: assert reading of legacy pickles against current data
While reviewing https://github.com/pandas-dev/pandas/pull/61770, I noticed that we didn't actually compare the read pickle data to some ground truth expected value, but just to itself (we were essentially doing `assert_equal(result, result)` ..), due to some accidental change in a clean-up many years ago in https://github.com/pandas-dev/pandas/commit/f2246cfa215d01b68aebd2da4afb836d912d248d)

Fixing that here by again creating the expected unpickled data with `create_pickle_data()` during the test run, to compare with the data from the older pickled files.
can you merge main and see if the pyarrow decimal issue resolves itself? This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this.