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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=h1) Report
> Merging [#19149](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9d8dbefb4248018f782b7df9d47adacc06edabea?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19149/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19149      +/-   ##
==========================================
- Coverage   91.53%   91.51%   -0.03%     
==========================================
  Files         148      148              
  Lines       48783    48783              
==========================================
- Hits        44654    44643      -11     
- Misses       4129     4140      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.88% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.61% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19149/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/19149/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.06% <0%> (+0.08%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=footer). Last update [9d8dbef...40a7c6a](https://codecov.io/gh/pandas-dev/pandas/pull/19149?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Added commits removing a no-longer-needed "xfail", adding a missing "assert_series_equal", and breaking up/parametrizing some giant tests. thanks! pls do the split of test_operators.py next