Added using-constant-test message example (#6591)

Author: matusvalo

doc/data/messages/u/using-constant-test/bad.py

@@ -0,0 +1,4 @@
+if 0:  # [using-constant-test]
+    print('This code is never executed.')
+if 1:  # [using-constant-test]
+    print('This code is always executed.')

doc/data/messages/u/using-constant-test/good.py

@@ -0,0 +1 @@
+print('This code is always executed.')