Improve logging of violations in test mode

[EndangeredMassa]

This PR extracts the violation formatting logic into a util class that can be used in both component audit and test audit scenarios.

It also changes the violationsHelper so that it works more like a singleton and is used throughout the project as a module instead of a global. It's still exposed as a global in the initializer, though. This also makes it work properly in test mode.

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Here's a before/after of using ember-a11y-testing in another project's test suite:

Before:

After:

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Here's a screenshot of development mode to show that it still works fine:

[dbollinger]

Looks good-- you are a saint! Any reason not to continue outputting the total number of violations?

[EndangeredMassa]

I can put that back in somewhere. What do we like?
"[critical] (x10): Buttons must have discernable text..."?

[EndangeredMassa]

Added a violation count to the error message. It now looks like:

[serious]: Elements must have sufficient color contrast 
Violated 5 times. Offending nodes are: 
<span class="c-radio-track__item-node">None</span>
<span class="c-radio-track__item-node">Level 1</span>
<span class="c-radio-track__item-node">Level 2</span>
<span class="c-radio-track__item-node">Level 3</span>
<p data-test-selector="labeless-text-input" id="violations__low-contrast-text" class="p-violations__grid-item-content--low-contrast-text c-paragraph ember-view">          Swoooosh
</p>
https://dequeuniversity.com/rules/axe/2.6/color-contrast?application=axeAPI
{object}

I also changed Offending markup is to Offending nodes are because what follows is a list of every node that violated that rule.

[MelSumner]

This is a really useful change. Thank you for doing this.

[JoaoGFarias]

Thank you! This improvement makes checking and debugging a11y in Ember apps so much faster!