Clean up tests to avoid `FutureWarning: ChainedAssignmentError`

[leostimpfle]

There are many tests that trigger the avoidable FutureWarning: ChainedAssignmentError which is caused by assigning a value to a copy of a pandas.DataFrame (see, e.g. here which is caused by this line). It would be nice to update the affected tests to reduce the amount of clutter in the test output.