RFR: 8278262: JFR: TestPrintXML can't handle missing timestamps

[Erik Gahlin]

Could I have review of a test fix. The timestamp, i.e. field "until" in the Thread Park event, is converted to an Instant value before comparing it to the XML value. This doesn't work when the value is missing (Long.MIN_VALUE). 

Testing: running jdk/jdk/jfr/tool/TestPrintXML.java 300 times without failure. 

Thanks
Erik

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Commit messages:
 - Initial

Changes: https://git.openjdk.java.net/jdk/pull/8681/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8681&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278262
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8681.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8681/head:pull/8681

PR: https://git.openjdk.java.net/jdk/pull/8681