[SalesForce] Trigger upsert questions

Multi-select picklists are stored as value1;value2;value3;etc. Knowing that, you can get a List of these values using the String split() method. You can also view this string splitting example. To access it, you would then tweak your code as:

trigger AutoCreateInterviewer on Position__c (after insert) {
    List<Interviewer__c> interviewers = new List<Interviewer__c>();

    //For each position processed by the trigger, add a new  

    //interviewer record for the specified hiring manager.  

    //Note that Trigger.New is a list of all the new positions  

    //that are being created.  

    for (Position__c newPosition: Trigger.New) {
        if (newPosition.Hiring_Manager__c != null) {
            // split out the multi-select picklist using the semicolon delimiter
            for(String hiringManager: newPosition.Hiring_Manager__c.split(';')){
                interviewers.add(new Interviewer__c(
                        Name = '1',
                        Position__c = newPosition.Id,
                        Employee__c = (Id)hiringManager,
                        Role__c = 'Managerial'));
            }
        }
    }
    insert interviewers;
}

@isTest
public with sharing class AutoCreateInterviewerTriggerTest {

    static testMethod void testGetAdvisor() {
        // First, set up your test User.
        User testUser = generateTestUser();

        // Generate your position but don't perform DML yet
        Position__c pos = new Position(Hiring_Manager__c = testUser.Id);

        // Start your test and insert your position
        Test.startTest();
        insert pos;
        Test.stopTest();

        // Get the interviewers from the DB to ensure they were created
        List<Interviewer__c> interviewers = [SELECT
                                                 Id
                                             FROM
                                                 Interviewer__c
                                             WHERE
                                                 Position__c = :pos.Id
        ];

        // There should be 1 interviewer because only one user ID was provided
        System.assert(1, interviewers.size(), 'One interviewer object is expected');
    }

    private User generateTestUser(){
        // We use the System Admin profile because it will always be there
        Profile sysAdmin = [SELECT 
                               Id 
                            FROM 
                               Profile 
                            WHERE 
                               Name = 'System Administrator'
                            LIMIT 1
        ];

        // Generate the user
        User newUser = new User(
            UserName = 'unit.tester@example.com',
            LastName = 'Test',
            FirstName = 'Jane',
            Email = 'unit.tester@example.com',
            phone = '555-555-5555',
            MobilePhone = '555-555-5554',
            Street = '123 Fake Street',
            City = 'Los Angeles',
            State = 'CA',
            PostalCode = '12345',
            CommunityNickName = 'unit.test',
            Alias = 'abcd',
            ProfileId = sysAdmin.Id,
            emailencodingkey = 'UTF-8',
            languagelocalekey = 'en_US',
            localesidkey = 'en_US',
            timezonesidkey = 'America/Los_Angeles'
        );

        // Insert that user
        insert user;

        return user;
    }
}

The Subsidiaries_On_Contract__c field needs to be populated and in addition have some values separated by a comma.

Eg abc,def

The code is trying to split based on the comma and creating a record for each part.

So check that the Subsidiaries_On_Contract__c field is populated.

You can set up debug monitor using Setup > Monitoring > Debug Logs

Add the name of the executing user, perform the save, and then come and look at the debug logs to see if there is any activity. (Worth adding some debugs in the trigger to print out say Subsidiaries_On_Contract__c - so System.debug('Subsidiaries_On_Contract__c ' + newContract.Subsidiaries_On_Contract__c); within the for loop)