[9] RFR: 8079138: Additional negative tests for XML signature processing

[Artem Smotrakov]

Hello,

Please review a couple of negative tests which check that invalid XML 
digital signatures are rejected.

Bug: https://bugs.openjdk.java.net/browse/JDK-8079138
Webrev: http://cr.openjdk.java.net/~asmotrak/8079138/webrev.01/

Artem