test: use promises-es6-tests for additional promise tests

The promises-es6-tests suite is a spin off of work that I did for @getify 's getify/native-promises-only module.

Eventually the goal with the tests is to convert them to Test262 format and submit them for review and inclusion in Test262 for ECMAScript.

At present they are a set of tests in mocha, using a style and runner very similar to promises-aplus (in fact, stolen from promises-aplus/promises-tests ), but covering ES6-draft-specified features such as Promise.all, Promise.race, etc.

es6-shim promises pass all the tests currently in promises-es6-tests.

/cc @ljharb
npm did that, but I have changed it.  npm install --save apparently uses "^" since early 2014.