RFR: JDK-8299520: TestPrintXML.java output error messages in case compare fails

[Matthias Baesken]

Currently the compare function in the jfr related test TestPrintXML.java does not output anything in case of failing event comparisons. This should be enhanced.

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Commit messages:
 - JDK-8299520

Changes: https://git.openjdk.org/jdk/pull/11827/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299520
  Stats: 12 lines in 1 file changed: 11 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11827.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11827/head:pull/11827

PR: https://git.openjdk.org/jdk/pull/11827