TST: Collect/Use arithmetic test fixtures
Takes over from #22350 (which would have been a rebasing nightmare), addressing AFAICT all unaddressed comments from there.

All remaining fixtures in tests/arithmetic are collected in tests/arithmetic/conftest.py
Fixtures are given more descriptive names and docstrings as requested in #22350.

A handful of tests are altered to use fixtures; in a number of cases their `pytest.mark.parametrize` arguments were already equivalent to one of the fixtures.

A few new broken cases are identified, specifically numeric `Series` and `DataFrame` `__mul__` or `__rdiv__` with a) non-nanosecond `timedelta64` or b) `Tick`.  (I'll open an Issue for these)

Some tests got cleaned up nicely by using `tm.assert_equal` instead of parametrizing with `assert_func`
Hello @jbrockmendel! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/conftest.py`](https://github.com/jbrockmendel/pandas/blob/d935afc98af15452c7eccea006a80756d14ccff0/pandas/tests/arithmetic/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_datetime64.py`](https://github.com/jbrockmendel/pandas/blob/d935afc98af15452c7eccea006a80756d14ccff0/pandas/tests/arithmetic/test_datetime64.py) !

 - In the file [`pandas/tests/arithmetic/test_numeric.py`](https://github.com/jbrockmendel/pandas/blob/d935afc98af15452c7eccea006a80756d14ccff0/pandas/tests/arithmetic/test_numeric.py), following                are the PEP8 issues :
> [Line 154:19](https://github.com/jbrockmendel/pandas/blob/d935afc98af15452c7eccea006a80756d14ccff0/pandas/tests/arithmetic/test_numeric.py#L154): [W504](https://duckduckgo.com/?q=pep8%20W504) line break after binary operator


 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_period.py`](https://github.com/jbrockmendel/pandas/blob/d935afc98af15452c7eccea006a80756d14ccff0/pandas/tests/arithmetic/test_period.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_timedelta64.py`](https://github.com/jbrockmendel/pandas/blob/d935afc98af15452c7eccea006a80756d14ccff0/pandas/tests/arithmetic/test_timedelta64.py) !



##### Comment last updated on September 10, 2018 at 01:57 Hours UTC >In the file pandas/tests/arithmetic/test_numeric.py, following are the PEP8 issues :

>> Line 154:19: W504 line break after binary operator

setup.cfg has W503 ignored for this this warning.  Why does pep8speaks think its W504? Traditional resourceWarning in circleCI # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22645?src=pr&el=h1) Report
> Merging [#22645](https://codecov.io/gh/pandas-dev/pandas/pull/22645?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/0976e1261781153603064eb67d66630a099edcd7?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/22645/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22645?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22645   +/-   ##
=======================================
  Coverage   92.17%   92.17%           
=======================================
  Files         169      169           
  Lines       50708    50708           
=======================================
  Hits        46740    46740           
  Misses       3968     3968
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.58% <ø> (ø)` | :arrow_up: |
| #single | `42.35% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22645?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/22645?src=pr&el=footer). Last update [0976e12...d935afc](https://codecov.io/gh/pandas-dev/pandas/pull/22645?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks. my comments are for a future PR FYI