Followup Cleanup DTI test_arithmetic, ASV
Remove ASV for Timestamp.offset since that attribute has been removed.

tests.indexes.datetimes.test_arithmetic has a giant offsets test that this splits up into reasonably sized pieces.
 - one piece of that test currently only runs if `klass is Series`, but that sub-test is now valid for DatetimeIndex too, so this PR removes that condition.
 - parametrize tests that currently iterate over a dictionary of inputs.
 - A mistaken-looking indentation in that iteration ATM prevents a bunch of cases from running.  This PR fixes that.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
