Shrink subtest can end up with oom killing it. Create subprocess
and run it from there so it will at least get reported like:

dynamic child 0 pid:70254 died with signal 9, Killed
Subtest shrink: FAIL (23.906s)

dmesg reports:

[103335.337309] Out of memory: Killed process 70254 (i915_suspend)

Cc: Riana Tauro <riana.tauro@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Reviewed-by: Sai Gowtham Ch <sai.gowtham.ch@intel.com>