I am new to Apex, and the testing has me a bit confused as to how to perform certain types of tests:

Below is my code, can some one help me see how to write a test for this as well as answer the questions below?

1. How to test a method that is defined as a PageReference? (i.e how to CALL it in a test method?)

2. When creating test data "on the fly" how long does the test data stay available (i.e If I create test data in the FIRST test method,  is the data gone right after that test completes or is that test data available to all the rest of the test methods I have defined. So Can I just add the test data ONCE, for ALL my tests or do I need to do it for EACH test method I have defined?

public PageReference processViewHousehold() {
 
        //Householdis is a PUBLIC variable contains list of id's
        if(householdid != null)
        {
            List<Account> HouseHold = new List<Account>(
            [
            Select id                    
            from account
            where payout__Households__c = :householdid
        
            ]
            );
        
        
           List<Id> myClientIdList = new List<id> ();
       
           for(Account a : HouseHold){
               myClientIdList.add(a.id);
           }       
   
           ClientIdList = myClientIdList;
        }
        
      
        return null;
    }