[3.12] gh-101100: Fix class reference in library/test.rst (GH-114769) (GH-114794)

The text clearly seems to be referencing `TestFuncAcceptsSequencesMixin`,
for which no target is available. Name the class properly and suppress
the dangling reference.
(cherry picked from commit 7a93db44257c0404dc407ff2ddc997f4bb8890ed)

Co-authored-by: Skip Montanaro <skip.montanaro@gmail.com>