Enabling a "false is empty" style check.

This is based off of pull request #10 from @athieriot.
Thanks for the idea!  I've implemented it as a flag and an environment
variable that can be enabled.  I've also added another test to handle
the environment variable.

run-tests

@@ -8,6 +8,7 @@ FAIL=0
 
 for TEST in tests/*.expected; do
     BASE="${TEST%.expected}"
+    MO_FALSE_IS_EMPTY=
 
     echo -n "$BASE ... "