implement timedeltas.test_scalar_compat
Centralize and split redundant TDI.shift tests in test_arith

Hello @jbrockmendel! Thanks for updating the PR.









Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 02, 2018 at 02:48 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19503?src=pr&el=h1) Report
> Merging [#19503](https://codecov.io/gh/pandas-dev/pandas/pull/19503?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/78ba063f438924378f887d77fcc692186e6381a6?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/19503/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19503?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19503      +/-   ##
==========================================
+ Coverage   91.67%    91.7%   +0.02%     
==========================================
  Files         148      148              
  Lines       48553    48553              
==========================================
+ Hits        44513    44525      +12     
+ Misses       4040     4028      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.06% <ø> (+0.02%)` | :arrow_up: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19503?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19503/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/19503?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/19503?src=pr&el=footer). Last update [78ba063...6ec7e5e](https://codecov.io/gh/pandas-dev/pandas/pull/19503?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!