Test calls igt_fork() so for i915 requires offset allocation arbitration

(allocator in multiprocess mode) especially when same drm fd is used

in children. Dedicated thread (intel_allocator_multiprocess_start())

is required to be started on the very beginning to handle offset

allocations as well as stopping it (intel_allocator_multiprocess_stop())

before test exits.

Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>

Cc: Luciano Coelho <luciano.coelho@intel.com>

Cc: Swati Sharma <swati2.sharma@intel.com>