RFR: 8326006: Allow TEST_VM_FLAGLESS to set flagless mode

[Leonid Mesnik]

Allow TEST_VM_FLAGLESS to enable/disable flagless tests execution explicitly.
Sometimes the tests are executed in the mode that is incompatible with flag-sensitive tests. This mode should be able to disable them explicitly.

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Commit messages:
 - 8326006: Allow TEST_VM_FLAGLESS to set flagless mode

Changes: https://git.openjdk.org/jdk/pull/17886/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17886&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326006
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17886.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17886/head:pull/17886

PR: https://git.openjdk.org/jdk/pull/17886