Add warning that no Cython-specific tests should be added to the PEP-492...

Add warning that no Cython-specific tests should be added to the PEP-492 coroutine test file copied from CPython.

tests/run/test_coroutines_pep492.pyx

@@ -2,6 +2,11 @@
 # mode: run
 # tag: pep492, pep530, asyncfor, await
 
+###########
+# This file is a copy of the corresponding test file in CPython.
+# Please keep in sync and do not add non-upstream tests.
+###########
+
 import re
 import gc
 import sys