report(test): split out axe test

[paulirish]

no changes other than splitting this test into its own file.

Since this thing takes 40s+, I personally want it separate so I can know that the REST of report-renderer-test is passing without waiting for this bigguy.

and while the yarn unit-report --watch will still be slow.... with this change, i'll happily run this instead: yarn jest $(find report -iname "*-test.js" | grep -v axe) --watch.

(of course, i'm also hoping to move this test to a karma/mocha setup at some point, but i don't have the time for that atm)

[connorjclark]

was really hoping jest simply had a skipTestCase flag, but lacking that, this change is fine.

[brendankenny]

(of course, i'm also hoping to move this test to a karma/mocha setup at some point, but i don't have the time for that atm)

I mentioned one way back in #9421 (comment): if we're willing to put yarn build-sample-reports in the prereqs for smoke tests, we have a far better way to run axe against reports than using jsdom (which only supports a subset of the real report in a real browser anyways), it'll be faster than 40s, and we can easily also audit the error report, reports in other locales, maybe extend it to catch perf regressions of the report, etc :)