tests: Add gem_exec_whisper

A more complicated store variant to stress inter-engine dependencies
(i.e. semaphores and sync). We write a control value from one batch into
the next and then execute it. This is repeated a few times with each
execution happening on a different engine (so the kernel has to
serialise operations between engines) until we finally write the value
out into our scratch buffer where we can check the result, just like a
Chinese whisper.

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

tests/Makefile.sources

@@ -41,6 +41,7 @@ TESTS_progs_M = \
 	gem_exec_store \
 	gem_exec_suspend \
 	gem_exec_reloc \
+	gem_exec_whisper \
 	gem_fenced_exec_thrash \
 	gem_fence_thrash \
 	gem_fence_upload \