RFR: 8287008: Improve tests for thread dumps in JSON format [v3]

[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.

Alan Bateman has updated the pull request incrementally with one additional commit since the last revision:

  Remove spurious colon

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8784/files
  - new: https://git.openjdk.java.net/jdk/pull/8784/files/15c2fd7b..4f9590f3

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8784&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8784&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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