TST: Move plotting related tests to tests/plotting
- [x] related to #13579
- [x] tests passed
- [x] passes `git diff upstream/master | flake8 --diff`

Because `test_graphics` is getting huge, split them based on data types under `tests/plotting` to make visualization related tests easier. 

@sinhrks Looks good!

While we are in splitting modus, maybe also split the tests for 'other' dataframe/series methods (hist/boxplot) on the one hand, and the 'misc' functions on the other hand?
 And travis is failing because of lint issues
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13621?src=pr) is **84.38%**

> Merging [#13621](https://codecov.io/gh/pydata/pandas/pull/13621?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will not change coverage

``` diff
@@             master     #13621   diff @@
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  Files           142        142          
  Lines         51235      51235          
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits          43235      43235          
  Misses         8000       8000          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last updated by [c9a27ed...89c8b13](https://codecov.io/gh/pydata/pandas/compare/c9a27ede0925ddbaa8d3ec9efd3c332a636505cf...89c8b137e9c943d1bb22479681932425114cca85?src=pr)
 Updated to use following file names:
- `.plot` (previously `test_graphics`)
  - `test_series`
  - `test_frame`
  - `test_groupby`
- others (previously `test_graphics_others`)
  - `test_hist_method`
  - `test_boxplot_method`
  - `test_misc`
 @sinhrks this is going to conflict with #13147 

If you prefer I can rebase after we merge this, or is other way around ok?
 I'm willing to rebase after #13147:)
 Question: do we actually want it in `tests/plotting` or in `plotting/tests` ? We seem to vary a bit between different submodules (eg `tseries`, `tools`, `io` have their tests inside the module, which I pesonally like)
 Whichever possible, `pandas/tools/tests/plotting` or `pandas/tests/plotting`. Newly created submodules (like `indexes`, `types` ) have its tests under `pandas/tests/`. 
 I actually meant `pandas/plotting/tests` (in the understanding that you were planning to move the plotting code to `pandas/plotting` in another PR). 
I personally like that tests are close the the implementation, certainly for things that are rather independent modules (like `io`, and which is the case for plotting as well I think).
 Yeah it'll be a goal if there is no objections. Shall we move `pandas/plotting/tests` in this PR (leaving the submodule itself empty), then move actual scripts in #13579 (split for review purpose)?
 Would like to decide the (temporary) namespace to work on. I'm +1 for `pandas/tests/plotting` for now, and moving `pandas/plotting/tests` once ready (as moved files are easy to compare different from splitting).
 That's fine for me. This is ready to merge then?
 @jorisvandenbossche yes, related files are not updated after the previous rebase.
 Merging then!
