gh-96021: Explicitly tear down the IsolatedAsyncioTestCase loop in tests (GH-96135) (GH-96235)

Tests for IsolatedAsyncioTestCase.debug() rely on the runner be closed
in __del__. It makes tests depending on the GC an unreliable on other
implementations. It is better to tear down the loop explicitly even if
currently there is no a public API for this.
(cherry picked from commit 4de06e3c)

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