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>

Lib/unittest/test/test_async_case.py

@@ -14,10 +14,10 @@ def tearDownModule():
 class TestAsyncCase(unittest.TestCase):
     maxDiff = None
 
-    def tearDown(self):
+    def setUp(self):
         # Ensure that IsolatedAsyncioTestCase instances are destroyed before
         # starting a new event loop
-        support.gc_collect()
+        self.addCleanup(support.gc_collect)
 
     def test_full_cycle(self):
         class Test(unittest.IsolatedAsyncioTestCase):
@@ -108,6 +108,7 @@ async def on_cleanup(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioLoop)
         try:
             test.debug()
         except MyException:
@@ -143,6 +144,7 @@ async def on_cleanup(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioLoop)
         try:
             test.debug()
         except MyException:
@@ -178,6 +180,7 @@ async def on_cleanup(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioLoop)
         try:
             test.debug()
         except MyException:
@@ -219,6 +222,7 @@ async def on_cleanup2(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioLoop)
         try:
             test.debug()
         except MyException:
@@ -331,6 +335,7 @@ async def cleanup(self, fut):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioLoop)
         try:
             test.debug()
         except MyException: