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.

Lib/test/test_unittest/test_async_case.py

@@ -43,10 +43,10 @@ async def __aenter__(self):
 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):
@@ -151,6 +151,7 @@ async def on_cleanup(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioRunner)
         try:
             test.debug()
         except MyException:
@@ -186,6 +187,7 @@ async def on_cleanup(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioRunner)
         try:
             test.debug()
         except MyException:
@@ -221,6 +223,7 @@ async def on_cleanup(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioRunner)
         try:
             test.debug()
         except MyException:
@@ -262,6 +265,7 @@ async def on_cleanup2(self):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioRunner)
         try:
             test.debug()
         except MyException:
@@ -424,6 +428,7 @@ async def cleanup(self, fut):
 
         events = []
         test = Test("test_func")
+        self.addCleanup(test._tearDownAsyncioRunner)
         try:
             test.debug()
         except MyException: