bpo-41620: TestCase.run() now always return a TestResult instance (GH-28030) (GH-28065)

Previously it returned None if the test class or method was
decorated with a skipping decorator.

Co-authored-by: Iman Tabrizian <iman.tabrizian@gmail.com>
(cherry picked from commit 7e246a3a7b43762480ee4fe0cfb859e8e997a8c8)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>