Demonstrate that plain unittest does not support async tests (#7607)

Co-authored-by: Ran Benita <ran@unusedvar.com>

testing/example_scripts/unittest/test_unittest_plain_async.py

@@ -0,0 +1,6 @@
+import unittest
+
+
+class Test(unittest.TestCase):
+    async def test_foo(self):
+        assert False

testing/test_unittest.py

@@ -1241,3 +1241,18 @@ def test_asynctest_support(testdir):
     testdir.copy_example("unittest/test_unittest_asynctest.py")
     reprec = testdir.inline_run()
     reprec.assertoutcome(failed=1, passed=2)
+
+
+def test_plain_unittest_does_not_support_async(testdir):
+    """Async functions in plain unittest.TestCase subclasses are not supported without plugins.
+
+    This test exists here to avoid introducing this support by accident, leading users
+    to expect that it works, rather than doing so intentionally as a feature.
+
+    See https://github.com/pytest-dev/pytest-asyncio/issues/180 for more context.
+    """
+    testdir.copy_example("unittest/test_unittest_plain_async.py")
+    result = testdir.runpytest_subprocess()
+    result.stdout.fnmatch_lines(
+        ["*RuntimeWarning: coroutine * was never awaited", "*1 passed*"]
+    )