Since commit 26f13cbf,
gegl_buffer_sample() has gotten much slower for non-nearest
samplers; the corresponding tests in perf/test-samplers take
forever as a result.  Let's simply disable them (note that the
tests for gegl_sampler_get() and gegl_sampler_get_fun(), which is
what performance-critical code should use anyway, are still enabled
for all samplers.)