Fix test checking /system in chroot in clean-up script.

Test that "$ART_TEST_CHROOT/system" is a directory, not a regular
file.

Test: adb shell rm -rf "$ART_TEST_CHROOT/system" && art/tools/cleanup-buildbot-device.sh
Bug: 34729697
Change-Id: I35279e93963b046f57099b8f0d52aa6a0020ab5e
diff --git a/tools/cleanup-buildbot-device.sh b/tools/cleanup-buildbot-device.sh
index 2144b02..ca5219a 100755
--- a/tools/cleanup-buildbot-device.sh
+++ b/tools/cleanup-buildbot-device.sh
@@ -40,7 +40,7 @@
   #
   # TODO: Reorder ART Buildbot steps so that "device cleanup" happens
   # before "setup device" and remove this special case.
-  adb shell test -f "$ART_TEST_CHROOT/system" \
+  adb shell test -d "$ART_TEST_CHROOT/system" \
     "&&" find "$ART_TEST_CHROOT/system" \
       ! -path "$ART_TEST_CHROOT/system/etc/selinux/plat_property_contexts" \
       ! -type d \