i915/i915_suspend: run in subprocess to catch oom

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>

tests/i915/i915_suspend.c

@@ -176,15 +176,22 @@ test_shrink(int fd, unsigned int mode)
 	void *mem;
 
 	gem_quiescent_gpu(fd);
-	igt_purge_vm_caches(fd);
 
-	mem = igt_get_total_pinnable_mem(&size);
-	igt_assert(mem != MAP_FAILED);
+	igt_multi_fork(child, 1) {
+		fd = gem_reopen_driver(fd);
+		igt_purge_vm_caches(fd);
 
-	igt_purge_vm_caches(fd);
-	igt_system_suspend_autoresume(mode, SUSPEND_TEST_NONE);
+		mem = igt_get_total_pinnable_mem(&size);
+		igt_assert(mem != MAP_FAILED);
 
-	munmap(mem, size);
+		igt_purge_vm_caches(fd);
+		igt_system_suspend_autoresume(mode, SUSPEND_TEST_NONE);
+
+		munmap(mem, size);
+		close(fd);
+	}
+
+	igt_waitchildren();
 }
 
 static void