[JDK-9] RFR: 8050371: MessageDigest tests

[zaiyao liu]

Hi all,

Please help to review those 2 new tests to be added for MessageDigest, 
New tests are added to address following:

- Fixed length hash value is returned with different data inputs
- Same hash value is returned with different update and digest methods

JDK issue: https://bugs.openjdk.java.net/browse/JDK-8050371
Webrev:    http://cr.openjdk.java.net/~zailiu/8050371/webrev/

Thanks

Kevin