[css] Fix indentation issue in SCSS mode with mixin variables (for #1846)

Fixes the issue in #1846 and auto-resets the indentation level when parentheses and curly braces are present, which should help indentation issues be less bothersome when an issue does present itself in a block.

In the second commit I modified the mode testing code a bit to allow for testing indentation, and added a couple small tests for CSS/SCSS modes. This should help to avoid regressions in the future. But, as usual, feel free to alter, add to, or ignore the second commit.