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.
Inspired by bug 7362 (painting a glitz surface onto an xlib surface
crashes cairo) and the lack of coverage for
_cairo_paginated_surface_acquire_source_image(), these tests attempt
to use each backend as a source surface for all the other backends.
For example, this checks that one can construct a PS file ready for
printing and then copy that surface to an image/xlib for previewing.
