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.
Otherwise the reference image (user-font-image-ref.png) gets
interpreted as an image-specific reference image for the
user-font test case resulting in a bogus failure.
