Junits for FrameworkUtil and BundleContext

We have a common Utility class, which has following method 

public static ResourceResolver getServiceResourceResolver(String subServiceName){

       final BundleContext bundleContext = FrameworkUtil.getBundle(Util.class).getBundleContext();
       final ServiceReference factoryReference = bundleContext
                                                                          .getServiceReference(ResourceResolverFactory.class.getName());
       final ResourceResolverFactory resolverFactory = (ResourceResolverFactory) bundleContext
                                                                                     .getService(factoryReference);

       final Map<String, Object> map = Collections.singletonMap(ResourceResolverFactory.SUBSERVICE,
                                                             (Object) serviceName);
-       return resolverFactory.getServiceResourceResolver(map);

}

This common Utility returns the resolver for AEMService : 

resolver = Util.getServiceResourceResolver("user");

While writing Junits for the AEMService, the resolver is always null, because JUnit tests run in a plain JVM with no OSGi Framework. Therefore there is no BundleContext available.

How do we write Junit for the AEMService.java