CLN: refactor tests outside of class setup in `style/test_matplotlib.py`
This is a refactor to remove the pytest class setup structure, reverting to class-less fixtures.
It also parametrises some of the tests.
No test is altered or removed.

```
def test_background_gradient(styler):
def test_background_gradient_color(styler):
def test_background_gradient_axis(styler, axis, expected):
def test_text_color_threshold(cmap, expected):
def test_background_gradient_vmin_vmax():
def test_background_gradient_int64():
def test_background_gradient_gmap_array(styler_blank, axis, gmap, expected):
def test_background_gradient_gmap_array_raises(gmap, axis):
def test_background_gradient_gmap_dataframe_align(styler_blank, gmap, subset, exp_gmap):
def test_background_gradient_gmap_series_align(styler_blank, gmap, axis, exp_gmap):
def test_background_gradient_gmap_wrong_dataframe(styler_blank, gmap, axis):
def test_background_gradient_gmap_wrong_series(styler_blank):
```