RFR: 8261: Use test flag to skip JDP multicast tests in JolokiaTest

[Martin Skarsaune]

I just became aware that there is a new flag used to skip JDP tests. 
I previously added some env checking to skip similar multicast tests for Jolokia. 
Hence it would be a lot cleaner to use the new standardized mechanism for this.
Trying out whether this fixes [JMC-8261](https://bugs.openjdk.org/browse/JMC-8261)

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Commit messages:
 - Try to use new test flag to decide whether multicast is supported

Changes: https://git.openjdk.org/jmc/pull/588/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=588&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8261
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/588.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/588/head:pull/588

PR: https://git.openjdk.org/jmc/pull/588