TST: add test for reindexing rows with matching index uses shallow copy with CoW
xref https://github.com/pandas-dev/pandas/issues/49473

This was already working, but I think it was not yet explicitly tested (at least not in the specific `copy_view` tests). And this addresses a TODO note in https://github.com/pandas-dev/pandas/issues/49473
thx @jorisvandenbossche 