Make unittest loaders avoid loading test cases that are abstract base classes

Currently, loading a TestCase-derived subclass that is also an abstract base class like the following:

import unittest
from abc import ABC, abstractmethod

class FooBase(unittest.TestCase, ABC):
    @abstractmethod
    def test(self): ...

class Foo(FooBase):
    def test(self): pass

unittest.main()

would currently result in:

TypeError: Can't instantiate abstract class FooBase without an implementation for abstract method 'test'

This PR fixes the error by making the unittest loaders avoid loading such test cases and methods from them.

• Issue: unittest loaders should avoid loading test cases that are abstract base classes #120665