gh-100690: Raise an AttributeError when the assert_ prefix is forgotten when using Mock (#100691)

Mock objects which are not unsafe will now raise an AttributeError when accessing an
attribute that matches the name of an assertion but without the prefix `assert_`, e.g. accessing `called_once` instead of `assert_called_once`.

This is in addition to this already happening for accessing attributes with prefixes assert, assret, asert, aseert, and assrt.