[crac] RFR: 8351402: [CRaC] Use System.nanoTime() in TimedWaitingTest

[Timofei Pushkin]

Replaces `System.currentTimeMillis()` with `System.nanoTime()` in `TimedWaitingTest` since the former can, in theory, jump back and forth and that may lead to the test failures.

Also adds a diagnostic assert.

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Commit messages:
 - Add monotonicity assert
 - Use System.nanoTime() in TimedWaitingTest

Changes: https://git.openjdk.org/crac/pull/209/files
  Webrev: https://webrevs.openjdk.org/?repo=crac&pr=209&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351402
  Stats: 3 lines in 2 files changed: 1 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/crac/pull/209.diff
  Fetch: git fetch https://git.openjdk.org/crac.git pull/209/head:pull/209

PR: https://git.openjdk.org/crac/pull/209