This slightly idealises the behaviour of clients with the aim of
measuring the kernel overhead of different workloads. This test focuses
on the cost of relocating batchbuffers.

A trace file is generated with an LD_PRELOAD intercept around
execbuffer, which we can then replay at our leisure. The replay replaces
the real buffers with a set of empty ones so the only thing that the
kernel has to do is parse the relocations. but without a real workload
we lose the impact of having to rewrite active buffers.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>