RFR: 8298341: Ensure heap growth in TestNativeMemoryUsageEvents.java

[Stefan Johansson]

Please review this test fix.

**Summary**
The new test TestNativeMemoryUsageEvents.java sometimes fail. The reason is that the _NativeMemoryUsage_ and _NativeMemoryUsageTotal_ events want to share a single `NMTUsage` snapshot. To achieve this there is an `AgeThreshold` set to 50ms that is considered when sending the events. If the usage snapshot is new enough we don't refresh it, but reuse it. In the test this becomes problematic if the recording is shorter than 50ms, then both the beginChunk event and the endChunk event will use the same NMT data. 

This could be solved by using the feature currently reviewed in #11541 and this is the plan, see: [JDK-8298276](https://bugs.openjdk.org/browse/JDK-8298276). But since this is not yet done, I propose to fix the test for now by just adding a short sleep during the recording to ensure the endChunk event is delayed enough to cause a new NMT snapshot to be taken.

**Testing**
* Local testing with a larger `AgeThreshold` to see that a sleep really helps the problem.
* Mach5 testing to verify the fix is good there as well, this is currently running.

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Commit messages:
 - 8298341: Ensure heap growth in TestNativeMemoryUsageEvents.java

Changes: https://git.openjdk.org/jdk/pull/11579/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11579&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298341
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11579.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11579/head:pull/11579

PR: https://git.openjdk.org/jdk/pull/11579