RFR: 8292592: JFR test TestNative is not reliable due to low rate of sampling.

[Robbin Ehn]

Split TestNative into two different test, so native sampling can have a high frequency.

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Commit messages:
 - JFR test fixes

Changes: https://git.openjdk.org/jdk/pull/9915/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9915&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292592
  Stats: 210 lines in 3 files changed: 136 ins; 74 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9915.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9915/head:pull/9915

PR: https://git.openjdk.org/jdk/pull/9915