Fix test for negative `substr*`start

Doc sasys "If either argument is less than 0 or is NaN, it is treated as if it were 0."
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substring

Trusted MDN, didn't check spec itself.

Tested with:

 - node -e "console.log('test'.substr(-200))"
Merge pull request #20 from cesanta/negsubstr

Fix test for negative `substr*`start