[fix][test] Fix UnfinishedStubbing issue in AuthZTests

[lhotari]

Motivation

Mockito is not thread safe and modifying mocks in multi-threaded code will result in failures such as this one:

   [ERROR] Failures:
    [ERROR]   NamespaceAuthZTest.testMaxConsumersPerTopic:1537->setAuthorizationPolicyOperationChecker:183 » UnfinishedStubbing
    Unfinished stubbing detected here:
    -> at org.apache.pulsar.broker.admin.NamespaceAuthZTest.setAuthorizationPolicyOperationChecker(NamespaceAuthZTest.java:173)

    E.g. thenReturn() may be missing.
    Examples of correct stubbing:
        when(mock.isOk()).thenReturn(true);
        when(mock.isOk()).thenThrow(exception);
        doThrow(exception).when(mock).someVoidMethod();
    Hints:
     1. missing thenReturn()
     2. you are trying to stub a final method, which is not supported
     3. you are stubbing the behaviour of another mock inside before 'thenReturn' instruction is completed

Modifications

• refactor the test code to setup the mock once and switch the implementation dynamically

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

[lhotari]

depends on #22546 which isn't included in branch-3.0, dropping cherry-picking to branch-3.0 for now