1_MyClass.java

public with sharing MyClass {
  public KbManagerI kavManager;
  
  public MyClass() {
    // Default KbManager Implementation
    this(new MyKbManager());
  }

  // Allows for Constructor Injection, so we can change the implementation for our tests.
  public MyClass(KbManagerI km) {
    this.kavManager = km;
  }
  
  public void doThing(Id articleId, Boolean flagAsNew) {
    this.kavManager.publishArticle(articleId, flagAsNew);
  }
}

2_KbManagerI.java

// An interface that describes the method signatures in KbManager.PublishingService
public interface KbManagerI {
  void publishArticle(Id articleId, Boolean flagAsNew);
  //... add more methods as necessary
}

3_MyKbManager.java

// This class implements the interface. It is a pass through that our code will reference instead of the PublishingService
public inherited sharing class MyKbManager implements KbManagerI {
  public void publishArticle(Id articleId, Boolean flagAsNew) {
    // this implementation of KbManagerI actually calls the real PublishingService
    KbManager.PublishingService.publishArticle(articleId, flagAsNew);
  }
}

4_KbManagerMock.java

// This mock implements the same interface, so we can inject it into MyClass during MyClassTest
public inherited sharing class KbManagerMock implements KbManagerI {
  // Can Also have private/public members for MyClassTest to access at runtime.
  public Boolean isSuccess = false;
  public Boolean throwException = false;
  
  public void publishArticle(Id articleId, Boolean flagAsNew) {
    // this implementation of KbManagerI actually calls the real PublishingService
    // Do anything
    if (throwException) {
      // Throw some exception here, now you can easily test try/catch blocks that are in MyClass
    }
    // We called the method we wanted, and it finished executing. We can say this was a success. You could similarly give test data to the mock in the same way that we are working with the booleans.
    isSuccess = true;
  }
}

5_MyClassTest.java

@IsTest
private class MyClassTest {
  @TestSetup
  public void setup() {/* ... */}
  
  @IsTest
  static void doThing_givenNewArticle_shouldPublishNew() {
    KbManagerMock mockPublisher = new KbManagerMock();
    /*
    *  Optionally assign data to the mock now
    *  Then pass (re: inject) it to MyClass constructor
    */
    MyClass unit = new MyClass(mockPublisher);
    
    Test.startTest();
    
      unit.publishArticle("someValidArticleId", true);
    
    Test.stopTest();
    
    System.assert(mockPublisher.isSuccess, "The mock publish method should have marked this attribute true.");
  }
}