RFR: 8319931: GenShen: Increase no progress threshold for TestThreadFailure

[William Kemper]

The test walks to close to the edge of an out of memory cliff. Intermittent failures are caused by [JDK-8316632](https://bugs.openjdk.org/browse/JDK-8316632). Further investigation is needed for Genshen. The intention of this PR is just to increase the setting `ShenandoahNoProgressThreshold` for this test to pass.

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Commit messages:
 - Increase no progress threshold for generational test

Changes: https://git.openjdk.org/shenandoah/pull/356/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=356&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319931
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/356.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/356/head:pull/356

PR: https://git.openjdk.org/shenandoah/pull/356