Create a new TaskMonitor each time to avoid flaky tests.

[LALAYANG]

What is the purpose of this PR

• This PR cleans the state polluted by org.apache.hadoop.hbase.monitoring.TestTaskMonitor.testDoNotPurgeRPCTask.

• It may be better to clean state pollutions so that some other tests won't fail in the future due to the shared state pollution.

Reproduce the test failure

• Run the test twice in the same JVM.

Expected result:

• The tests should run successfully when multiple tests that use this state are run in the same JVM.

Actual result:

• We get the failure:
  [ERROR] Failures: [ERROR] testDoNotPurgeRPCTask:144 RPC Tasks have been purged! expected:<10> but was:<20>

Why the test fails

• Each time this test runs,10 RPCtasks are created and added to the ArrayList rpcTasks, but rpcTasks is not completely purged when the test ends. So next time the test starts, the remaining RPC tasks lead to an assertion failure.

Fix

Create a new TaskMonitor each time to avoid pollution similar to the other tests (e.g., testTaskMonitorBasics, testTasksGetAbortedOnLeak) in TestTaskMonitor.