[jdk21u-dev] RFR: 8365919: Replace currentTimeMillis with nanoTime in Stresser.java

[Rui Li]

A clean backport for JDK-8365919.

Stress tests need monotonic time. Replace `System.currentTimeMillis` with `System.nanoTime`. Tests are running. Backport this for the parity with oracle.

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Commit messages:
 - Backport 0ca38bdc4d503158fda57bbc8bc9adc420628079

Changes: https://git.openjdk.org/jdk21u-dev/pull/2284/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2284&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365919
  Stats: 25 lines in 1 file changed: 6 ins; 6 del; 13 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/2284.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2284/head:pull/2284

PR: https://git.openjdk.org/jdk21u-dev/pull/2284