Improve Qunit tests

[adamsilverstein]

Summary

Fix QUnit test on travis-ci.
Tests was running asynchronously and travis is not failing when the test faild.

Addresses #111

Relevant technical choices

The issue where the build wasn't failing when it should. This was a known issue with the node-qunit-phantomjs package (jonkemp/node-qunit-phantomjs#26) and seems to be due to the proposed usage running the tests asynchronously. This causes the gulp task to finish "successfully" before the tests finish running. The solution here was to change it to run in a synchronous way.

Checklist

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 4.7 and PHP 5.4.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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[aaemnnosttv]

@adamsilverstein my changes can be summarized as:

• Running qunit synchronously
  This fixes the known issue of qunit not returning a failing error code if all tests don't pass.
  See Test runner doesn't return correct failure code if run in node script jonkemp/node-qunit-phantomjs#26
• Reverting change to showErrorNotification assertion
  This was failing because the assertion was assuming a development build, where a production build is used in Travis

Currently the build is failing due to visual tests.

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[felixarntz]

Looks good!