trigger CustomObject on CustomObject__c (after update) {
    EventBus.publish(new Platform_Event__e());
}

@isTest private CustomObjectTriggerTest() {
    static testMethod void testTrigger() {
        CustomObject__c rec = new CustomObject__c(Name = 'A');
        Test.startTest();
            rec.Name = 'b';
            update rec;
        Test.stopTest();
        //System.assert( ... ??? Is there anything here we can assert? )
    }
}

trigger CustomObject on CustomObject__c (after update) {
    CustomObjectService.publishEvent();
}

public class CustomObjectService() {
    @testVisible private static List<Platform_Event__e> eventList = new List<Platform_Event__e>();
    public static void publishEvent() {
        Platform_Event__e eve = new Platform_Event__e();
        eventList.add(eve);
        EventBus.publish(eve);
    }
}

@isTest private CustomObjectTriggerTest() {
    static testMethod void testTrigger() {
        CustomObject__c rec = new CustomObject__c(Name = 'A');
        Test.startTest();
            rec.Name = 'b';
            update rec;
        Test.stopTest();
        System.assert( 1, CustomObjectService.eventList.size(), 'There should be one element in the list' );
    }
}