TST: add CoW test for setitem with Series being set
xref https://github.com/pandas-dev/pandas/issues/49473

We already have a test for setitem from https://github.com/pandas-dev/pandas/pull/50692/, but that only handled setting an array (actually, does that copy the array?), and not setting with a pandas object for which we could use lazy copy mechanism. 

This is only adding the test, we seem to actually do an eager copy, so room for optimization.
https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.isetitem.html The comment in the test was a left-over from copying the previous setitem test, will remove.

But the docstring is actually confusing here, because it _is_ an inplace method in the sense that it is updating the _object_ inplace (we listed this method in the "always inplace" bucket in the draft PDEP). It's only not inplace in the sense that it will not update _values_ inplace.

 Yeah good point, a clarification would probably help Fixed a small typo/copy paste issue. otherwise good to merge on greenish thx @jorisvandenbossche 

failures were unrelated