PLOT: Split plotting tests
#26414 splitted the pandas plotting code into two different modules:
- `pandas.plotting`: Generic plotting framework that defined an API (with docs) and is able to plot with a selectable backend (will be selectable with a pandas option)
- `pandas.plotting._matplotlib`: Matplotlib backend

Our tests in `pandas/tests/plotting` are currently testing both together (calling the pandas plotting framework, which will call the matplotlib backend). But ideally we'd like to have two different groups of tests:
- One with the current code but calling the matplotlib backend directly
- A test set of the generic pandas plotting framework, using a mock backend (needs to be implemented)

@jreback let me know if this makes sense to you