Builds boot-test-harness.img, if INSTALLED_BOOTIMAGE_TARGET.

Similar to commit Ic887ea93d4c5181eca0f82c3cdf3ce3b72f4c185
for boot-debug.img, we should also only build boot-test-harness.img
if boot.img exists.

Bug: 184365242
Test: tree hugger

Change-Id: Ie0d9e460905d488e418cf4ee1bd44e6e7c58470d