`mplexporter` tests should run in CI (and failing tests should be fixed)

[emilykl]

Two mplexporter tests are failing on main:

FAILED plotly/matplotlylib/mplexporter/tests/test_basic.py::test_legend_dots - AssertionError: assert '3' == '2'
FAILED plotly/matplotlylib/mplexporter/tests/test_utils.py::test_linestyle - AssertionError: 

This is not reflected in the CI because we are not running the mplexporter tests in the CI.

Plausibly so long as we are continuing to support mplexporter functionality, we should be running the tests.