Centralize and de-duplicate comparison and arith tests
These are almost all cut/paste moving tests to the appropriate places.  Two exceptions:

- Some "TODO: de-duplicate this with that" notes (mostly to myself)
- `TestTimedeltaIndexArithmetic.test_numeric_compat` badly needed to be split into more specific tests, so that is done here.
Hello @jbrockmendel! Thanks for updating the PR.























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

##### Comment last updated on January 19, 2018 at 18:52 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19317?src=pr&el=h1) Report
> Merging [#19317](https://codecov.io/gh/pandas-dev/pandas/pull/19317?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f0cd23ca6485940e3a6572cc4f5afdce4c3921ab?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/19317/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19317?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19317      +/-   ##
==========================================
- Coverage   91.53%    91.5%   -0.03%     
==========================================
  Files         150      150              
  Lines       48738    48738              
==========================================
- Hits        44611    44599      -12     
- Misses       4127     4139      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.87% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19317?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19317/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.74%)` | :arrow_down: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19317?src=pr&el=footer). Last update [f0cd23c...77ffa9a](https://codecov.io/gh/pandas-dev/pandas/pull/19317?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks.

~~are we missing a pandas/tests/indexes/period/test_artithmetic.py?~~

actually there is one, I guess nothing to move there