Add test script
Add test script

This makes it easier for new developers to run the tests, by typing `npm
test`. This will build `xregexp-all.js`, then open `tests/index.html` in a
browser, using [opener].

[opener]: https://www.npmjs.com/package/opener

mathiasbynens/xregexp,mathiasbynens/xregexp,slevithan/XRegExp,GerHobbelt/xregexp,mathiasbynens/xregexp,slevithan/xregexp,GerHobbelt/xregexp,slevithan/XRegExp,slevithan/xregexp,mathiasbynens/xregexp,slevithan/xregexp,GerHobbelt/xregexp