[CLEANUP] Use spaces for indenting the fixture CSS files

[oliverklee]

This makes editing the fixture files a bit easier as all other files in this project also use spaces for indentation, not tabs.

Note: This has no effect on CSS is rendered (which still defaults to tabs at the moment).

[coveralls]

coverage: 42.261%. remained the same
when pulling a6b5c12 on task/fixture-spaces-instead-of-tabs
into 8cd39f4 on main.

[JakeQZ]

Some tests might be intended to confirm that the parser can handle alternative whitespace forms, but that doesn't seem to be the case for any of these, not even whitespace.css.