Implement missing tests, fix offset floordiv, rfloordiv, fix return types
This splits off of #19365 a bunch of peripheral changes that were never intended to be part of #19365 in the first place.

- Implements many tests in the "how is there not already a test for that!?" category
- Fixes(+tests) Timedelta // DateOffset and DateOffset // Timedelta
- Fixes(+tests) return types for `Timedelta.__add__, __radd__, __sub__, __rsub__(np.timedelta64)` (ATM returns `timedelta64`) 
- Fixes a typo in test_construction that causes a dummy assertion to be made instead of the intended assertion.

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19738?src=pr&el=h1) Report
> Merging [#19738](https://codecov.io/gh/pandas-dev/pandas/pull/19738?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/44c822de8dee0bf0e1ed2e8bc15424bb323b786f?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19738/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19738?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19738      +/-   ##
==========================================
+ Coverage   91.58%   91.61%   +0.02%     
==========================================
  Files         150      150              
  Lines       48862    48862              
==========================================
+ Hits        44750    44763      +13     
+ Misses       4112     4099      -13
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19738?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19738/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `97.01% <0%> (+0.17%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19738/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `66.95% <0%> (+1.73%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19738?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/19738?src=pr&el=footer). Last update [44c822d...edcd7e0](https://codecov.io/gh/pandas-dev/pandas/pull/19738?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Closing n favor of #19752 plus  followups