Unit tests for LESS/SCSS

Also fixes for #8894 and #8895.
Good question! I just filed an issue (#8945) last night with `@include ... { ...}`. So I tested it with `@include` block and didn't see any issue in live highlighting. But when I debug it, I'm seeing a selector array like `[".sidebar", "@include breakpoint(large)"]. Luckily, `_getSelectorInFinalCSSForm(selectorArray)` function does strip at rules and the returns `.sidebar ` for my example. Even though there's no visible issue for end users, we still need to fix it here by excluding all at-rules mistakenly collected as selectors. Nice catch!