releasetools: Add tests for VerityImageBuilder.

`testdata/verity_mincrypt` is generated with the following command.
  $ generate_verity_key \
        -convert testdata/verity.x509.pem \
        testdata/verity_mincrypt

Test: python -m unittest test_verity_utils
Change-Id: I3f2f0cd89da0fd5d2420e0e2b09761cc77f893c5