Fix failing PDF fontsize verbatim test.

The failure happens on unspecified behavior of `Dir.glob('*')` order.
The assertion can only pass for input files which contain a `\code{}` macro,
the only one of which is a_chapter.tex.
Merge branch 'fix-pdf-test' of https://github.com/cirosantilli/softcover into cirosantilli-fix-pdf-test
Merge branch 'cirosantilli-fix-pdf-test'
Merge branch 'master' of https://github.com/softcover/softcover