update assert messages in materials_test.py

[jvavrek]

This shows that the extra messages are of the form:

FAILED tests/materials_test.py::test_materials_dummy_compendium_2022 - AssertionError: Expected no MaterialsWarnings to be raised; got ["Passing literal html to 'read_html' is deprecated and will be removed in a future version. To read from a literal string, wrap it in a 'StringIO' object.", "Passing literal html to 'read_html' is deprecated and will be removed in a future version. To read from a literal string, wrap it in a 'StringIO' object."]