[3.12] gh-121039: add Floats/ComplexesAreIdenticalMixin to test.support.testcase (GH-121071) (#123841)

* [3.12] gh-121039: add Floats/ComplexesAreIdenticalMixin to test.support.testcase (GH-121071)
(cherry picked from commit 8ef8354ef15e00d484ac2ded9442b789c24b11e0)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>