Avoid calling libtool to link every single test case, by building just one
binary from all the sources.
This binary is then given the task of choosing tests to run (based on user
selection and individual test requirement), forking each test into its own
process and accumulating the results.
The problem showed up on OS X because the freetype backend reuses font_face_t's
which kept the reference count high enough for long enough to avoid the problem.
The combination of the initial cairo_paint() and the translucent text
colors were causing image fallbacks that prevented the PS type 3 font
embedding from being tested.
The ability to draw glyphs with different metrics is useful when doing
font substitution with fixed layout like in pdf and I eventually plan on
adding code to poppler to do something similar.
