RFR: 8342646: JTREG_TEST_THREAD_FACTORY in testing.md should be TEST_THREAD_FACTORY

[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.

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Commit messages:
 - 8342646: JTREG_TEST_THREAD_FACTORY in testing.md should be TEST_THREAD_FACTORY

Changes: https://git.openjdk.org/jdk/pull/21594/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21594&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342646
  Stats: 5 lines in 4 files changed: 0 ins; 0 del; 5 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