css parser: handle single quotes for property values

And modify one of the testcases where there are already two double-quoted strings, so that one of them is single-quoted. This way make check fails without the code change.

Hopefully this is trivial enough -- I'm considering orcus's CSS parser to test libepubgen's CSS output, but that one always uses single quotes for font names, so orcus currently fails on that (as far as I see valid) input.