(fix #2097) PGF: get fonts from fc-list, use builtin fonts for tests

Attempt to fix #2097 where `font_manager.findSystemFonts()` includes matplotlib bundled non-system fonts which are unavailable to latex. Instead, rely on the fonctonfig `fc-list` tool which is assumed to be present on Linux and OSX systems.
While this is probably safe for Linux, we still have to find out if this is a safe assumption for OSX.