RFR: JDK-8314426: runtime/os/TestTrimNative.java is failing on slow machines

[Thomas Stuefe]

Patch hardens the test for slow machines:

- remove the grep for "Native Trimmer Stop" message, since that message is raced by the VM death and may not appear on slow machines
- for the low-interval test, made the number of trims to happen dependent on the runtime of the test. On slow machines, we could run longer than 3 seconds and therefore see more trim messages

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Commit messages:
 - JDK-8314426-runtime/os/TestTrimNative.java-is-failing-on-ppcle-aarch64

Changes: https://git.openjdk.org/jdk/pull/15309/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314426
  Stats: 7 lines in 1 file changed: 4 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15309.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15309/head:pull/15309

PR: https://git.openjdk.org/jdk/pull/15309