dma-buf: add selftest for fence order after merge

Add a test which double checks that fences are in the expected order
after a merge.

While at it also switch to using a mock array for the complex test
instead of a merge.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241115102153.1980-5-tursulin@igalia.com