RFR: 8306825: Monitor deflation might be accidentally disabled by zero intervals

[Aleksey Shipilev]

Found this mistake while backporting [JDK-8305994](https://bugs.openjdk.org/browse/JDK-8305994): if we put GuaranteedAsyncDeflationInterval=0 (defined as "disable guaranteed deflation"), then the MonitorDeflationThread would call `wait(0)`, that is, wait indefinitely for the notify. This breaks triggering the threshold heuristics!

But what is even more concerning, the same thing would happen even prior to [JDK-8305994](https://bugs.openjdk.org/browse/JDK-8305994), if we put GuaranteedSafepointInterval=0 too. Which means users who kept GSI=0 as the way to dodge excess safepoints, now effectively disable monitor deflation!

New test cases capture most of the interesting paths/combinations. Since I needed to add more test cases, I ended up refactoring the test for more clarity. The test is also renamed, because now it tests not only GADI, but other intervals too.

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/13635/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13635&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306825
  Stats: 621 lines in 4 files changed: 385 ins; 231 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/13635.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13635/head:pull/13635

PR: https://git.openjdk.org/jdk/pull/13635