jmx-dev RFR: 8350818: Improve OperatingSystemMXBean cpu load tests to don't accept -1.0 by default

[Leonid Mesnik]

The cpuLoad tests were updated to fail if -1.0 returns to catch bugs like https://bugs.openjdk.org/browse/JDK-8350820

The -1.0 means that JDK can't obtain cpu load. It shouldn't be returned.
If this functionality doesn't work on certain configurations then they should be excluded.

The fix should be pushed after https://bugs.openjdk.org/browse/JDK-8350820
I filed separate PR to backport fix easier.
Also, I haven't changed indentation and didn't change getSystemCpuLoad to getCpuLoad. They return same. 
Might be it would be better. just to rename the whole test later.

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Commit messages:
 - testfix

Changes: https://git.openjdk.org/jdk/pull/23833/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23833&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350818
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/23833.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23833/head:pull/23833

PR: https://git.openjdk.org/jdk/pull/23833