specify varnames in test_codeobject

Makes the test compatible with CPython 3.7.3 which would otherwise raise
an exception: ValueError("code: varnames is too small")

Author: msimacek

graalpython/com.oracle.graal.python.test/src/tests/cpyext/test_codeobject.py

@@ -69,7 +69,7 @@ def compile_module(self, name):
                 1, 2,
                 3, 4, 0,
                 b"", tuple(), tuple(),
-                tuple(), tuple(), tuple(),
+                ("a", "b", "c"), tuple(), tuple(),
                 "filename", "name", 1,
                 b"",
             ),