RFR: 6661: Simplify Agent JMX tests

[Jessye Coleman-Shapiro]

This patch simplifies mbean function calls for agent JMX tests.  

The proposed solution, which is outlined in the ticket's description, was to create an mbean object by calling ManagementFactory.newPlatformMXBeanProxy(...). However this function requires the  AgentControllerMBean class to be loaded by the platform class loader, which it is not.  So instead I have called the JMX.newMXBeanProxy(...) directly.

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Commits:
 - 646146d7: Simplify Agent JMX tests

Changes: https://git.openjdk.java.net/jmc/pull/37/files
 Webrev: https://webrevs.openjdk.java.net/jmc/37/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6661
  Stats: 8 lines in 1 file changed: 1 ins; 3 del; 4 mod
  Patch: https://git.openjdk.java.net/jmc/pull/37.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/37/head:pull/37

PR: https://git.openjdk.java.net/jmc/pull/37