Although adding the line:

 Dev_Package_RNController obj = new  Dev_Package_RNController();

will call the appropiate section of code, there is more you need to add to your code.  Essentially what you have to remember first of all is that when writing test code you should never assume that there is data in your database.  So you should create a RN__c and RN_Payment_Line_Item__c and insert them into the database.  So you should add the following:

RN__c RN1 = new RN__c(field1Name=field1Value, etc);

RN_Payment_Line_Item__c RNPLI1 = new RM_Payment_Line_Item__c(some list);

insert RN1;

insert RNPLI1;

In addition your code calls ApexPages.currentPage().getParameters().get('id') so you need to set the current page of reference.  For this you could use the following:

PageReference pageRef = new PageReference('/apex/myPage?id='+ RN1.id);
Test.setCurrentPageReference(pageRef);

Finally, it is good practice to assert that your code performed as desired with something like:

system.assertEquals (value1, value2);

So putting it all together your code should look something like:

@isTest
 
private class Dev_Package_RNController_Test{
 
static testmethod void testPackageController()
   {
ApexPages.StandardController con = new ApexPages.StandardController(new rn__c());

RN__c RN1 = new RN__c(field1Name=field1Value, etc);
RN_Payment_Line_Item__c RNPLI1 = new RM_Payment_Line_Item__c(some list);
insert RN1;
insert RNPLI1;

PageReference pageRef = new PageReference('/apex/myPage?id='+ RN1.id);
Test.setCurrentPageReference(pageRef);
 
     test.startTest();  //start actual testing
     Dev_Package_RNController Control  = new Dev_Package_RnController(con); 
     Dev_Package_RNController Control1 = new Dev_Package_RnController(); 

     string returnedName = control.getname();
     RN__c returnedRN = control.getRN();
     RN_Payment_Line_Item__C returnedRNPLI = control.getRNpli();

     test.stopTest();  //testing complete

//below this you should re-query both of the objects that you inserted 
//above and then assert that they equal the ones from above 
//for example

system.assertEquals(returnedName, 'Dev_Package_RNController');

//and so on 

   
          }
}

 Obviously there are some values to change and I will leave it to you to finish the assert statements, but I hope this helps.