RFR: 8340969: jdk/jfr/startupargs/TestStartDuration.java should be marked as flagless

[Leonid Mesnik]

Test jdk/jfr/startupargs/TestStartDuration.java
checks duration, the time might be too small for stress options like Xcomp.
So it makes sense to mark it as flaglesss.

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Commit messages:
 - 8340969: jdk/jfr/startupargs/TestStartDuration.java should be marked as flagless

Changes: https://git.openjdk.org/jdk/pull/21237/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21237&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340969
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21237.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21237/head:pull/21237

PR: https://git.openjdk.org/jdk/pull/21237