Add the is_unexpected key to the test output.

If the key is missing, it seems the failure is assumed to be expected.

BUG=473924

Change-Id: I62edd9110fa74bee5e6425fd6786badf5398728c
Reviewed-on: https://boringssl-review.googlesource.com/4231
Reviewed-by: Adam Langley <agl@google.com>