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When running the cairo-test-suite, valgrind reports each of the
registered test as a leak, because they are _list_prepend()'ed, but
the tests list is never _list_free()'d.

Fixes the following valgrind error:

malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
cairo_boilerplate_xmalloc (cairo-boilerplate-system.c:47)
cairo_test_register (cairo-test-runner.c:131)
_cairo_test_runner_register_tests (cairo-test-constructors.c:1112)
main (cairo-test-runner.c:714)
Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>