Implement arithmetic.test_numeric and arithmetic.test_object
Also move a bunch more dt64 and td64 tests that have been hiding.  For the most part this doesn't do any of the parametrizing over box-classes; that and a bunch of de-duplication are for the next pass.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22254?src=pr&el=h1) Report
> Merging [#22254](https://codecov.io/gh/pandas-dev/pandas/pull/22254?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/8206c369a90b7cb1f9f4816944406a3af68bdb93?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22254/graphs/tree.svg?token=eZ4WkYLtcO&width=650&src=pr&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22254?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22254   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files         169      169           
  Lines       50683    50683           
=======================================
  Hits        46666    46666           
  Misses       4017     4017
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <ø> (ø)` | :arrow_up: |
| #single | `42.34% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22254?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/22254?src=pr&el=footer). Last update [8206c36...d255a6b](https://codecov.io/gh/pandas-dev/pandas/pull/22254?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks