RFR: 8287008: Improve tests for thread dumps in JSON format [v5]

[Alan Bateman]

> This change is mostly test infrastructure and improvements to the testing of thread dumps in JSON format. It also tweaks the thread dump format so that the process identifier and thread identifiers are a type string rather than number.
> 
> The tests for thread dumps added by JEP 425 are changed to use the test infrastructure library. The tests for JEP 428 will also make use of this test infrastructure.

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

  Review comments

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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/353fc53d..7e0c1513

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8784&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8784&range=03-04

  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 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