Trigger that is inserting the contact ID of an associated contact via Opportunity contact role into custom field on opportunity

trigger UpdateOppConvertContactTrigger on Opportunity (before update) {

    List<OpportunityContactRole> ContactRoles = [SELECT OpportunityId, ContactId
    FROM OpportunityContactRole];

    Map<String, OpportunityContactRole> OppIdToContactMap = new Map<String, OpportunityContactRole>();
    for (OpportunityContactRole ConRole : ContactRoles) {
        OppIdToContactMap.put(ConRole.OpportunityId, ConRole);
    }

    for (Opportunity opp : Trigger.new) {
        if (opp.Opportunity_Contact__c == NULL) {
            OpportunityContactRole newContact = OppIdToContactMap.get(opp.Id);
            opp.Opportunity_Contact__c = newContact.ContactId;
            opp.Type = 'New Customer';
        }
    }
}

Base test 

@isTest
public class BaseTest {

    public static Account newAccount(String name) {
        return new Account(
                Name = name,
                BillingStreet = 'Some street',
                BillingCity = 'Some City',
                Email__c = 'steve@example.com',
                Phone = '0123456789',
                Industry = 'Agriculture'
        );
    }

    public static Contact newContact(String firstName, String lastName, String accountId) {
        return new Contact(
                FirstName = firstName,
                LastName = lastName,
                Phone = '0123456789',
                Email = 'steve@example.com',
                AccountId = accountId
        );
    }

    public static Opportunity newOpportunity(String Oppname, String accountId) {
        return new Opportunity(
                AccountId = accountId,
                Name = Oppname,
                StageName = 'Appointment Booked',
                LeadSource = 'Test',
                Lead_Source_Specific__c = 'Other',
                CloseDate = Date.newInstance(2018, 12, 09)
        );
    }

}

Test class

@IsTest
public class UpdateOppConvertContactTriggerTest {

    @IsTest
    static void testOppInsert() {
        Account testAccount = BaseTest.newAccount('name');
        insert testAccount;

        Contact testContact = BaseTest.newContact('first', 'last', testAccount.Id);
        insert testContact;

        Opportunity testOpportunity = BaseTest.newOpportunity('oppname', testAccount.Id);
        insert testOpportunity;

        OpportunityContactRole testRole = new OpportunityContactRole();
        testRole.OpportunityId = testOpportunity.Id;
        testRole.ContactId = testContact.Id;
        insert testRole;

        update testOpportunity;

        System.assertEquals(testContact.Id, testRole.ContactId);
        System.assertEquals(testContact.Id, testOpportunity.Opportunity_Contact__c);
    }
}

I am getting 100% coverage without the System.assertEquals at the end. However, it keeps failing with ID in the field being Null. How do i find out why my trigger is not firing? is the update method correct that should fire the trigger? 

much appeciated