Mocking AEM Asset Manager using WCM IO

I am creating a sling model which fetched a excel file from the file to read and display data in an AEM component.

@Model(adaptables = SlingHttpServletRequest.class, defaultInjectionStrategy = DefaultInjectionStrategy.OPTIONAL)  
public class OnlineCoursesModel {  
     @Self 
     private SlingHttpServletRequest request;  

     @ValueMapValue 
     private String filePath;  

     private List<OnlineCourseDTO> onlineCourses;  

     @PostConstruct 
     public void init() { 
          AssetManager assetManager = request.getResourceResolver().adaptTo(AssetManager.class); 
          Asset asset = assetManager.getAsset(filePath);  

          /** Do Something With the Asset **/  
     } 
}

In AEM its working fine, but when I try to use it with the wcm io AEM mocking framework, the assetManager is returning null.

@Test
public void checkIfFileIsRead() {
  context.load().binaryFile(COURSES_EXCEL_FILE, EXCEL_RESOURCE_PATH);
  context.load().json(ONLINE_COURSE_LISTING_AUTHORED, TEST_CONTENT_ROOT);
  resource = context.request();
  undertest = resource.adaptTo(OnlineCoursesModel.class);

  System.out.println(undertest);
}