Refactor: listen to onload for stylesheets in before hook of tests

[jeeyyy]

This PR refactors the addStyleSheet helper for CSSOM based tests into axe.testUtils object. The function also returns a promise, thereby allowing to wait until the stylesheets are loaded in the before hook, thus ensuring assets are available before running respective tests.

Closes issue:

• Refactor: tests with external stylesheets (PR #1132) #1086

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[jeeyyy]

Meh! Had setup gitconfig to not recognise user, hence CLA was not passing. Had altered gitconfig and created a new PR with fresh commit of same changes. Hence closing this.