Use android_root instead of hard coded /system in test script.

The buildbot uses its own ANDROID_ROOT.

Spotted while debugging 902 test failures on target.

Change-Id: I9b2047ce510e93f205771f7384f644450b0325e6
diff --git a/test/run-test b/test/run-test
index 250263a..c50a9ca 100755
--- a/test/run-test
+++ b/test/run-test
@@ -476,7 +476,7 @@
         run_args="${run_args} --runtime-option -Djava.library.path=${ANDROID_HOST_OUT}/lib${suffix64}:${ANDROID_HOST_OUT}/nativetest${suffix64}"
     else
         guess_target_arch_name
-        run_args="${run_args} --runtime-option -Djava.library.path=/data/nativetest${suffix64}/art/${target_arch_name}:/system/lib${suffix64}"
+        run_args="${run_args} --runtime-option -Djava.library.path=/data/nativetest${suffix64}/art/${target_arch_name}:${android_root}/lib${suffix64}"
         run_args="${run_args} --boot /data/art-test/core${image_suffix}${pic_image_suffix}${multi_image_suffix}.art"
     fi
     if [ "$relocate" = "yes" ]; then