gh-96021: Explicitly close the IsolatedAsyncioTestCase runner in tests (GH-96135)

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 close the runner explicitly even if
currently there is no a public API for this.
(cherry picked from commit 4de06e3cc0a58d73934f9a2759ad9cd2f6b031b0)

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