RFR: 8342646: JTREG_TEST_THREAD_FACTORY in testing.md should be TEST_THREAD_FACTORY [v2]

[SendaoYan]

> Hi all,
> In [make/RunTests.gmk](https://github.com/openjdk/jdk/blob/master/make/RunTests.gmk#L208), the keyword is 'TEST_THREAD_FACTORY'.
> 
> So the below test command will print error:
> 
> make test TEST=test/jdk/java/math/BigInteger/TestValueExact.java CONF=linux-x86_64-server-release JTREG="JTREG_TEST_THREAD_FACTORY=Virtual"
> 
> 
> Building target 'test' in configuration 'linux-x86_64-server-release'
> JTREG_TEST_THREAD_FACTORY=Virtual is not a valid keyword for JTREG.
> Valid keywords: JOBS TIMEOUT_FACTOR FAILURE_HANDLER_TIMEOUT TEST_MODE ASSERT VERBOSE RETAIN TEST_THREAD_FACTORY MAX_MEM RUN_PROBLEM_LISTS RETRY_COUNT REPEAT_COUNT MAX_OUTPUT REPORT OPTIONS JAVA_OPTIONS VM_OPTIONS KEYWORDS EXTRA_PROBLEM_LISTS LAUNCHER_OPTIONS.
> RunTests.gmk:206: *** Cannot continue. Stop.
> 
> 
> So I think we should fix the document bug in `doc/testing.md` to avoid take the same mistake. Trivial fix, no risk.

SendaoYan has updated the pull request incrementally with two additional commits since the last revision:

 - JTREG="TEST_THREAD_FACTORY=Virtual"
 - Use JTREG="JTREG_TEST_THREAD_FACTORY=Virtual" to make comment more cleaner

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21594/files
  - new: https://git.openjdk.org/jdk/pull/21594/files/f1aab5dc..7c6a7bb4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21594&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21594&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21594.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21594/head:pull/21594

PR: https://git.openjdk.org/jdk/pull/21594