trigger updatestageoncontact on Interview__c(after insert, after update){
 list<Id> contIds = new list<Id>();
 list<Contact> Contacts = new list<Contact>();
 for(Interview__c inte:trigger.new){
  if(inte.Status__c=='Scheduled'){
    contIds.add(inte.Candidate__c);
  }
 }
 if(!contIds.isEmpty()){
  for(contact con:[select Id, Stage__c from contact where Id IN :contIds]){
    con.Stage__c='Interview Scheduled';
    Contacts.add(con);
  }
    update contacts;
 }
}

@IsTest
private without sharing class testupdatestageoncontact {

   @TestSetup
   static void makeData() {
       Account acc = new Account(Name = 'Test Acc');
       Insert Acc;
       Contact con = new Contact(FirstName = 'Test', LastName = 'Contact', AccountId = 
       acc.Id);
       Insert con;
       //Put in whatever status is accepted that isn't Scheduled
       Interview__c int = new Interview__c(Status__c = 'Pending', Candidate__c = con.Id);
       Insert int; 
   }
   @isTest
    static void testupdateStage() {
       Interview Int = [SELECT Id, Status__c FROM Interview__c WHERE Status__c = 
      'Pending' LIMIT 1];
      Test.startTest();
      Int.Status__c = 'Scheduled';
      update Int;
      Test.stopTest();
      System.assertEquals(1, [SELECT Count() FROM Contact WHERE Stage__c = 
      'Interview Scheduled']);
    }