[jdk21u-dev] RFR: 8326006: Allow TEST_VM_FLAGLESS to set flagless mode

[Amos Shi]

Backport of [JDK-8326006](https://bugs.openjdk.org/browse/JDK-8326006)

Testing
- Local: Not Applicable.
- Pipeline: 
- Testing Machine:

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

Commit messages:
 - Backport d10f277bd39bb5ac9bd48939c916de607fef8ace

Changes: https://git.openjdk.org/jdk21u-dev/pull/502/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=502&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326006
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/502.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/502/head:pull/502

PR: https://git.openjdk.org/jdk21u-dev/pull/502