RFR: 8292407: Improve Weak* VarHandle tests resilience under spurious failures

[Aleksey Shipilev]

We have a few reports that existing Weak* VarHandle tests are still flaky, for example on large AArch64 machines or small RISC-V machines.

The flakiness is intrinsic to the nature of Weak* operations under tests, that can spuriously fail. The last attempt to fix these was [JDK-8155739](https://bugs.openjdk.org/browse/JDK-8155739). We need to strengthen these a bit more.

The actual values depend on the successful testing on known-failing platforms. I ballparked bumping the attempts 5x and introducing the delay would help without exploding test time in worst cases.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/9889/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9889&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292407
  Stats: 297 lines in 26 files changed: 296 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/9889.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9889/head:pull/9889

PR: https://git.openjdk.org/jdk/pull/9889