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
1 file changed