Fixturize tests/frame/test_arithmetic
Split off from #22730 as per review from @WillAyd 

The changes in `conftest.py` are due to the following:
> In translating the quasi-fixtures from `TestData` to `conftest` in #22236, I sorted the dtypes for the columns of `mixed_float_frame` and `mixed_int_frame`, which turns out to have been a mistake. This is reverted here to be a true translation of the attribute of `TestData`. Otherwise, tests in the newly fixturized `test_arithmetic.py` would fail.
Hello @h-vetinari! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/tests/frame/conftest.py`](https://github.com/h-vetinari/pandas/blob/3ee3b709f7132cbf480c14c36c4abe947ca62be9/pandas/tests/frame/conftest.py) !

 - There are no PEP8 issues in the file [`pandas/tests/frame/test_arithmetic.py`](https://github.com/h-vetinari/pandas/blob/3ee3b709f7132cbf480c14c36c4abe947ca62be9/pandas/tests/frame/test_arithmetic.py) !



##### Comment last updated on September 20, 2018 at 10:06 Hours UTC @h-vetinari ping when updated @jreback 

I though your request for `all_arithmetic_operators` would not change anything, but it includes more operators than previously tested - I get errors for all the reverse operators:
```
AttributeError: module 'operator' has no attribute '__rmod__'  [etc.]
```

I've skipped those tests now. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22736?src=pr&el=h1) Report
> Merging [#22736](https://codecov.io/gh/pandas-dev/pandas/pull/22736?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1c113db60b68c5a262d64e92dc9de72bfe59aed5?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22736/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22736?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22736      +/-   ##
==========================================
+ Coverage   92.17%   92.18%   +0.01%     
==========================================
  Files         169      169              
  Lines       50778    50819      +41     
==========================================
+ Hits        46807    46850      +43     
+ Misses       3971     3969       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (+0.01%)` | :arrow_up: |
| #single | `42.37% <ø> (+0.04%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22736?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.82% <0%> (-0.21%)` | :arrow_down: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `98.35% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <0%> (ø)` | :arrow_up: |
| [pandas/core/dtypes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Jhc2UucHk=) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.67% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.34% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.2% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.45% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/excel.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZXhjZWwucHk=) | `97.4% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.75% <0%> (+0.01%)` | :arrow_up: |
| ... and [11 more](https://codecov.io/gh/pandas-dev/pandas/pull/22736/diff?src=pr&el=tree-more) | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22736?src=pr&el=footer). Last update [1c113db...b0b82a7](https://codecov.io/gh/pandas-dev/pandas/pull/22736?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jreback Green ok by me. @jbrockmendel if you'd have a look and merge if ok. thanks @h-vetinari 

so this is a good example of a change. you moved / parameterized things, but its a single change that is quite straightforward for a reviewer to grok / comment on w/o diving into things too much.