test: throw a helpful error when running alongside the extension
test: throw a helpful error when running alongside the extension

Currently running the `axe-core` test suite in a browser that has either the axe or attest extension enabled will yield unpredictable and confusing results. This patch causes the test suite to throw an error instructing people to disable their extension(s) before running their tests.

I ran into this while working on a feature and wasted ~1 day trying to debug. :'(

Side note - I'm opening a seperate PR into the extensions repository which exposes the `__AXE_EXTENSION__` global.`

dequelabs/axe-core,dequelabs/axe-core,dequelabs/axe-core,dequelabs/axe-core