Cleanup Series arithmetic tests
Also a few for TimedeltaIndex.  One of the TimedeltaIndex tests specifically tests a behavior I think is wrong, #18963.  Actually _fixing_ that bug is separate.

Actual content of tests is unchanged.
Travis error in TestTSPlot.test_time looks unrelated. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18974?src=pr&el=h1) Report
> Merging [#18974](https://codecov.io/gh/pandas-dev/pandas/pull/18974?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4883a432a21e65d998a57708e20196627f7c60d7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18974/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/18974?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18974      +/-   ##
==========================================
+ Coverage   91.58%   91.58%   +<.01%     
==========================================
  Files         150      150              
  Lines       48967    48939      -28     
==========================================
- Hits        44846    44823      -23     
+ Misses       4121     4116       -5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.95% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (+0.01%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18974?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/18974/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `90.2% <0%> (-0.04%)` | :arrow_down: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/18974/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.45% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/18974/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `92.92% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18974/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.57% <0%> (+0.12%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/18974/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.19% <0%> (+0.15%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18974/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.9% <0%> (+0.21%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18974?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/18974?src=pr&el=footer). Last update [4883a43...65b120d](https://codecov.io/gh/pandas-dev/pandas/pull/18974?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Test error still looks unrelated and this is bound to cause merge headaches for other higher-priority PRs.  Closing for now, will circle back eventually.