RFR: 8287008: Improve tests for thread dumps in JSON format

[Alan Bateman]

This is a test-only change to add some test infrastructure and improve the testing of thread dumps in JSON format. The new tests added by JEP 425 for this thread dump format search the JSON text for strings but don't parse it completely.  These tests can be improved with a test class that parses the thread dump. The tests for JEP 428 like make use of the test infrastructure too.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/8784/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8784&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287008
  Stats: 332 lines in 3 files changed: 305 ins; 9 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8784.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8784/head:pull/8784

PR: https://git.openjdk.java.net/jdk/pull/8784