MotionVectorTestLarge: reduce height on 32-bit system

Reduce height from 540 t0 360, hopefully to resolve the test failures
related to OOM.

BUG=aomedia:1177

Change-Id: Ia697d8d820a664e76f9a455189d4a58068591ddd

test/motion_vector_test.cc

@@ -82,7 +82,7 @@ TEST_P(MotionVectorTestLarge, OverallTest) {
   // Reduce the test clip's resolution while testing on 32-bit system.
   if (sizeof(void *) == 4) {
     width = 2048;
-    height = 540;
+    height = 360;
   }
 
   cfg_.rc_target_bitrate = 24000;