Add test for CSS imports.

We didn't have one at all!
3 jobs for css-processing in 7 minutes and 45 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #112503
fedora test

00:07:21

 
  Lint
manual #112505
allowed to fail manual
clippy
passed #112504
rustfmt

00:00:23