TST: add test for numpy ops, esp. nanmin/max bug for np<1.13
- [x] closes #8383
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The bug in #8383 concerns np.nanmin and np.nanmax only, don't know if I'm testing too much.
Comments addressed. Not sure if I understood your second, but please look at changed test functions. lgtm ping on green # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19753?src=pr&el=h1) Report
> Merging [#19753](https://codecov.io/gh/pandas-dev/pandas/pull/19753?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/5bb5e33ffe01eb7c82e8eed327f8745cfacdaada?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19753/graphs/tree.svg?height=150&src=pr&width=650&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19753?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19753      +/-   ##
==========================================
- Coverage    91.6%    91.6%   -0.01%     
==========================================
  Files         150      150              
  Lines       48864    48864              
==========================================
- Hits        44763    44761       -2     
- Misses       4101     4103       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.75% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19753?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19753/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19753?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/19753?src=pr&el=footer). Last update [5bb5e33...07b2995](https://codecov.io/gh/pandas-dev/pandas/pull/19753?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 all green. thanks @topper-123 