Test that bad Finished messages are rejected.

That's a pretty obvious thing to test. I'm not sure how we forgot that one.

Change-Id: I7e1a7df6c6abbdd587e0f7723117f50d09faa5c4
Reviewed-on: https://boringssl-review.googlesource.com/4211
Reviewed-by: Adam Langley <agl@google.com>