Add tests for imgdiff.

Factor out libimgdiff static library for testing purpose.

This CL adds the imgdiff tests on host and on target both (similar to
libimgpatch). In practice, we only need imgdiff binary on host, and
libimgpatch on target. But they should build and pass tests on both
platforms.

Test: recovery_host_test passes; recovery_component_test passes.
Change-Id: I0eafb7faf727cdf70066310e845af6ee245d4f60