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