In 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.