tests/micropython/const.py: Add comment about required config for test.

Expected result of const.py will be matched only when MICROPY_COMP_CONST is
enabled.  For easy understanding, added description at the first of the
test code.

tests/micropython/const.py

@@ -1,4 +1,5 @@
 # test constant optimisation
+# This test will only work when MICROPY_COMP_CONST is enabled.
 
 from micropython import const