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
diff --git a/tools/releasetools/verity_utils.py b/tools/releasetools/verity_utils.py
index acc7300..5343fa3 100644
--- a/tools/releasetools/verity_utils.py
+++ b/tools/releasetools/verity_utils.py
@@ -299,7 +299,7 @@
     """Creates an image that is verifiable using dm-verity.
 
     Args:
-      out_file: the location to write the verifiable image at
+      out_file: the output image.
 
     Returns:
       AssertionError: On invalid partition sizes.