[DO NOT MERGE] Normalise formatting of spec output

This PR is an extension of https://github.com/sass/sass-spec/pull/207 which normalises the output format of the `expected_output.css` files to the what Ruby sass 3.4.9 generates.

This change was originally introduced in https://github.com/sass/sass-spec/pull/206 but still has some kinks as mentioned in https://github.com/sass/sass-spec/pull/206#commitcomment-9097986.

IMHO this is a valuable change if we can iron out the current issues.