test/fuzz: fix a group of errors related to `for`

Running of automatically generated Lua programs sometimes failed
with an errors like:

- "'for' initial value must be a" (12%)
- "'for' limit must be a" (4.5%)
- "'for' step must be a" (0.4%)

The patch fixes these errors.

NO_CHANGELOG=testing
NO_DOC=testing
NO_TEST=testing