RFR: 8272908: Missing coverage for certain classes in com.sun.org.apache.xml.internal.security

[Fernando Guallini]

This patch improves code coverage on the following classes:
- com.sun.org.apache.xml.internal.security.algorithms.implementations.IntegrityHmac
- com.sun.org.apache.xml.internal.security.utils.ElementProxy
- com.sun.org.apache.xml.internal.security.keys.KeyInfo

The new tests exercise code blocks that are not currently covered by other tests.

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Commit messages:
 - add code coverage to KeyInfo and IntegrityHmac

Changes: https://git.openjdk.java.net/jdk/pull/5913/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5913&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272908
  Stats: 253 lines in 3 files changed: 246 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5913.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5913/head:pull/5913

PR: https://git.openjdk.java.net/jdk/pull/5913