I'm trying to get an opportunity to be sent to be approved when a checkbox is ticked (the checkbox will be ticked by various workflows).

This is the code I've cobbled together from various sources:

trigger OpportunitySubmitForApproval1 on Opportunity (after update) {

for (Opportunity a : trigger.new) {
if (a.Send_for_Approval__c == TRUE) {
    Approval.ProcessSubmitRequest app = new Approval.ProcessSubmitRequest();
    ID addendumId = a.Id;

      Approval.ProcessSubmitRequest request = new Approval.ProcessSubmitRequest();
            request.setObjectId(addendumId);
            Approval.ProcessResult requestResult = Approval.process(request);
  }
}
}


This is working fine for me in Sandbox. However, I'm having trouble making a test class to bring it over to production. The test class I've cobbled together is 
@isTest
public class OpportunitySubmitForApproval1Test {
    static testMethod void testApprovalSuccess() {

        Opportunity opp = new Opportunity();
        opp.Name = 'Test Opp';
        opp.Amount = 100;
        opp.CloseDate = Date.today();
        opp.Probability = 10;
        opp.StageName = 'Prospecting';
        opp.Send_for_Approval__c = FALSE;
        // insert the new opp
        insert opp;
        // change the probability of the opp so the trigger submits it for approval
    opp.Probability = 40;
        opp.Send_for_Approval__c = TRUE;
    // update the opp which should submit it for approval
    update opp;

        // ensure that the opp was submitted for approval
        List < ProcessInstance > processInstances = [select Id, Status from ProcessInstance where TargetObjectId = :opp.id];
    System.assertEquals(processInstances.size(),1);

    }


}