[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 7a93db4)

Co-authored-by: Skip Montanaro <[email protected]>

Author: miss-islington

Doc/library/test.rst

@@ -143,7 +143,7 @@ guidelines to be followed:
          arg = (1, 2, 3)
 
   When using this pattern, remember that all classes that inherit from
-  :class:`unittest.TestCase` are run as tests.  The :class:`Mixin` class in the example above
+  :class:`unittest.TestCase` are run as tests.  The :class:`!TestFuncAcceptsSequencesMixin` class in the example above
   does not have any data and so can't be run by itself, thus it does not
   inherit from :class:`unittest.TestCase`.