Shows object diff when an async assertion fails

Before, the assertion failure that mocha could readily interpret was
wrapped when inside an async hook. This made for very not helpful
failures.

This unwraps to prevent this.

See mochajs/mocha#2797

test/helper.js

@@ -3,6 +3,12 @@ const chai = require('chai');
 chai.config.includeStack = true;
 global.expect = chai.expect;
 
+// Otherwise assertion failures in async tests are wrapped, which prevents mocha from
+// being able to interpret them (such as displaying a diff).
+process.on('unhandledRejection', err => {
+  throw err;
+});
+
 if (process.env.TEST_SUITE === 'browser') {
   /* eslint-disable no-console */
   console.log('WARN: Skipping nodejs tests because TEST_SUITE is \'browser\'.');