TST: Split graphics_test to main and others
Because `test_graphics.py` gets little longer, I've splitted it to 2 files. Test methods itself are not changed.
- `test_graphics.py` is for `.plot()` methods.
- `test_graphics_others.py` is for other plots, such as `.hist()`, `.boxplot` and miscs. 

OK, this seems fine by me... though we do eventually probably want to consolidate all these methods into `.plot.hist()` and so forth (I promise I'll finish that PR!)
 @shoyer In plot accessor, hist and box calls `.plot(kind="hist")` and `.plot(kind="box")` internally. These tests are included in `test_graphics` because these are part of `.plot` method. So no worries.

Moved to "test_graphics_others.py" are  `DataFrame.hist` and `DataFrame.boxplot` which used prior to `.plot(kind="hist")`,  etc. These use separate (inconsistent) logic from `.plot`.
 Is this OK after v0.16.1 visualization-related PRs are merged (#8885)?
@jreback, @TomAugspurger
 Yeah. I have time set aside this weekend.  Will review then.
 @sinhrks status?
 @TomAugspurger  OK for this?
 Yeah, the only thing I would maybe add is a module docstring at the top of the file explaining which tests go where. But go ahead and merge.
 @TomAugspurger Thanks. Added short explanations to both files, and will merge after it gets green.
