Provide assertAccessible(element) for closure unit tests

[ewinslow]

I apologize for not having the time to submit a proper pull request, but I wanted to get this recorded because it's pretty straightforward and I'd like other projects to be able to take advantage of this:

/**
 * Asserts that the given element and its descendants pass a suite of
 * accessibility checks.
 *
 * @param {!Element} element
 */
function assertAccessible(element) {  
  var auditConfig = new axs.AuditConfiguration();
  auditConfig.scope = element;

  var results = axs.Audit.run(auditConfig);
  var auditResults = axs.Audit.auditResults(results);

  assert(auditResults.toString(), auditResults.numErrors() == 0);
}

[alice]

This is great, but I'm unsure where it should live in this project. I'd love to hear from someone using closure testing (e.g. @ewinslow if still applicable!) where it would be useful.

[ewinslow]

I was thinking a new directory like src/testing/closure.js Then you could also have src/testing/jasmine.js for jasmine, etc.

@ckundo recommended we make separate repos for the axs-tools-to-testing-library glue code. One per library. I'm leaning slightly against that since separate repos feels like it would incur a little extra overhead than necessary, given the size of the code we're talking about.

[ckundo]

@ewinslow yes, I agree with you now. It'd be less complicated than pointing to multiple repos. We're including the test runner in this repo, it makes sense that plugins would live here too.

[ewinslow]

Great. It'll be nice to have all those helpers under one roof...