feat: Upgraded unittests to ESLint 8.x

[bfanger]

CLIEngine is removed in ESLint 8
https://eslint.org/docs/8.0.0/user-guide/migrating-to-8.0.0

I've updated the tests and wrapped it in a async function (lintFiles is asynchronous)

Another fix to running the tests would be to modify the package.json:
change "eslint": ">=6.0.0", into "eslint": ">=6.0.0 <8",
(aka limit the eslint version to 7.x)

[dummdidumm]

Thanks. I decided to merge #155 which takes another approach by limiting this to eslint < 8. Reason: This plugin supports ESLint 6+ and with ESLint 8 used in dev we might accidentally introduce a breaking change if we bump to 8 now.