DOC: Undo test runner API doc hack #10478

Description

automodapi was replaced with autoclass for test runner API doc in #10477 . The suspect was an incompatibility between sphinx-automodapi and Sphinx 3.1.0 (also see astropy/sphinx-automodapi#105).

Expected behavior

No warning/exception should occur when using automodapi to render test runner API doc.

Actual behavior

#10473 happened and was causing CI failure in the html-docs job in unrelated PRs.

Steps to Reproduce

Undo the hack in #10477 and use Sphinx 3.1.0 or greater.