BUG: yt.funcs.matplotlib_style_context isn't tested

[neutrinoceros]

Bug report

Bug summary

This function hasn't received any care for a while, and it's currently not tested.
Currently, it doesn't produce the expected results

Code for reproduction

import matplotlib.pyplot as plt
import yt

with yt.funcs.matplotlib_style_context():
    fig, ax = plt.subplots()
    ax.plot([0, 1], [0, 1])
    ax.set(xlabel=r"$\Sigma$", ylabel=r"$\beta$", title="My fake yt plot")
    fig.savefig("/tmp/yt_style_funcs.png")

Actual outcome

Expected outcome

Something like

[neutrinoceros]

Actually yt.funcs.matplotlib_style_context is used internally in a couple of places, so it's not completely untested, but there are no unit tests for it.