ENH: tm.assert_(is|not)_copy, tm.shares_memory
There are a number of places in the tests where we check that a result is a copy, is not a copy, is a view, is not a view, etc.  We end up with ad-hoc implementations all over the place.  It'd be nice to have a something like `assert not tm.shares_memory(series, series.copy())`.  This would likely require EA authors to implement parts of it.