[SalesForce] Unit Tests: create completed tasks for ActivityHistory

Any chance your running user in the testmethod falls afoul of these restrictions (from SFDC Doc)

In order to prevent performance issues while still providing the related list functionality, there are some restrictions on users who do not have “View All Data” permission. Such users must comply with the following restrictions:

In the main clause of the relationship query, you can reference only one record. For example, you cannot filter on all records where the account name starts with A, but must reference a single account record.

You cannot use WHERE clauses.

You must specify a limit to the number of rows returned, less than 500.

You must sort on ActivityDate and LastModifiedDate, descending order: ORDER BY ActivityDate DESC, LastModifiedDate DESC

The trigger code is having lot of redundancy, So I modified it. before posting the modified code few lines of explanation what I changed. you collected account Ids twice and did the condition check twice where once is more enough. I am also made changes to the test class, where Task data is provided which is not required because the trigger has to create it once the necessary conditions are met. I am expecting this trigger should work for you, if there is any errors feel free to post as a comment.

trigger OpportunityAfter on Opportunity (after update){

    Set AccountIDs = new Set();

    for(Opportunity o : trigger.new){
        if(opp.StageName == 'Closed - Lost' && opp.Business_Line__c == 'Workforce' && 
            opp.Survey_Type__c != 'APulse' && trigger.oldMap.get(opp.Id).StageName != opp.StageName){

                AccountIDs.add(o.AccountId);
         }
    }

    Task[] tasksToInsert = new Task[]{};

    for (Account acc : [Select Id, Name, OwnerId from Account where Id IN :AccountIds]){
        for (Opportunity opp : trigger.new){
            if(opp.AccountId == acc.Id){
                Task tas = new Task();
                tas.WhatId=acc.Id;
                tas.OwnerId=opp.OwnerId;
                tas.Subject='Follow-up: ' + opp.name + ' - ' + acc.name;
                tas.ActivityDate=(opp.CloseDate + 180);
                tas.Description='Follow up to see how engagement initiatives are going and if Avatar can help. Refer to lost opportunity: ' + opp.name;
                tasksToInsert.add(tas);                                                         
            }
        }
    }
    if(tasksToInsert != null && !tasksToInsert.isEmpty())
    Database.insert(tasksToInsert);

}

@isTest
public class TestClass {

    static testMethod void myUnitTest() {

    //Set up user
    User u1 = [SELECT Id FROM User WHERE Email='abc@sampleemail.com'];
    test.startTest();  
    //Run As U1
    System.RunAs(u1){

    System.debug('Testing trigger... (single record validation)');

    //NEW Account record
    Account acc = new account();
    acc.Name = 'Test Account';
    acc.Industry = 'Healthcare';

    insert acc;

    //NEW Opportunity record
    Opportunity opp = new Opportunity();
    opp.Name = 'Test Opportunity'; 
    opp.CloseDate = System.today();
    opp.StageName = 'S1 - Investigative';
    opp.Type = 'Repeat Business - Wrkforce HC';
    opp.AccountId = acc.Id;
    opp.Survey_Type__c = 'Product Name';

    insert opp;

    opp.StageName = 'Closed - Lost';
    opp.Reason_Won_Lost__c = 'Relationship';
    opp.Business_Line__c = 'Workforce'  //added this line to meet the if condition while collecting the set of id in the trigger.

    update opp;
   test.stopTest();
    // Here I am expecting the task has been inserted and doing a query for it         and check it in the assert statement.
   Task  testTask=[Select Id, WhatId from Task where WhatId=:acc.Id];
    //Validate single insert
    System.assertEquals(acc.Id, testTask.WhatId);

    }
    }
}