Slack to Salesforce Integration using Apex Trigger, Test Class Not Covered

I am trying integrate Slack using Apex Trigger and not by process builder. I figured out a way to get it executed using Trigger and works perfectly for my requirements. However, I am unable to get a proper test coverage and the method is passed as well with no errors.

I am fairly new with this type of integrations. I am sure I am missing something which I am not able to figure out.

I have other methods too which I am calling from Apex Class, but figured if I can cover one method, I will be able to work with others. See below:

Apex Trigger:(50% - Test coverage):

trigger SalesforceToSlackOppTrigger on Opportunity (after insert, after update) {

Id NewRecordTypeId = Schema.SObjectType.Opportunity.getRecordTypeInfosByName().get('New').getRecordTypeId();

List<Id> opptyIds = new List<Id>();

if(SalesforceToSlack.isFirstTime) {
    for(Opportunity opp:Trigger.new){

        SalesforceToSlack.isFirstTime = false;
        Account acct = [SELECT Id, CRM_Owner__c FROM Account WHERE Id =: Opp.AccountId];
        if((opp.RecordTypeId == NewRecordTypeId && opp.isClosed == true && opp.isWon == True && acct.CRM_Owner__c == null) 
        && (opp.OwnerId != '00536000003AUNV' && opp.OwnerId != '00536000001i9UG')){
            opptyIds.add(opp.Id);
            if(opptyIds.size() > 0){
                SalesforceToSlack.OppWonPushToSlack(opptyIds);
            }
        }
    } 
}   

}

Apex Class (1% - Test coverage):

public class SalesforceToSlack { private static final String OppWonSlackURL = 'WEBHOOK_URL' //Edited out for privacy

public static void OppWonPushToSlack(List opportunityId) { Id oppId = opportunityId[0]; // If bulk, only post first to avoid overloading Slack channel Opportunity o = [SELECT Id, Name, CloseDate, Account.Name, Owner.Name FROM Opportunity WHERE Id=:oppId]; Map msg = new Map(); msg.put('text', 'Please assign CRM owner to our following new opportunity in salesforce:' + '\nOpportunity Name: ' + o.Name + '' + '\nCompany Name: ' + o.Account.Name + '' + '\nClose Date: ' + o.CloseDate +'' + '\nOpportunity Owner: ' + o.Owner.Name +'' + '\nLink: ' + 'https://cs14.salesforce.com/' + o.id); msg.put('mrkdwn', true); String body = JSON.serialize(msg);
System.enqueueJob(new QueueableSlackCall(OppWonSlackURL, 'POST', body)); }

public class QueueableSlackCall implements System.Queueable, Database.AllowsCallouts {

    private final String url;
    private final String method;
    private final String body;

    public QueueableSlackCall(String url, String method, String body) {
        this.url = url;
        this.method = method;
        this.body = body;
    }

    public void execute(System.QueueableContext ctx) {
        HttpRequest req = new HttpRequest();
        req.setEndpoint(url);
        req.setMethod(method);
        req.setBody(body);
        Http http = new Http();
        if (!Test.isRunningTest()) {
            HttpResponse res = http.send(req);
        }    
    }
}

}

Apex Test Class:

@isTest public class SalesforceToSlackTest {

static testMethod void testOppWonPushToSlack() {
Id NewRecordTypeId = Schema.SObjectType.Opportunity.getRecordTypeInfosByName().get('New').getRecordTypeId();
Id pricebookId = Test.getStandardPricebookId();

Account acct1 = new Account(Name = 'Test Account 1',
                            Company_Type__c = 'Trading',
                            Trading__c = 'Hedge Fund',
                            CRM_Owner__c = null);
Insert acct1;

Product2 prod = new Product2(Name = 'US',
                             isActive = true
                             );
insert prod;

//Create your pricebook entry
PricebookEntry pbEntry = new PricebookEntry(Pricebook2Id = pricebookId,
                                            Product2Id = prod.Id,
                                            UnitPrice = 10000,
                                            IsActive = true
                                            );
insert pbEntry;
Opportunity newOpp = new Opportunity(Name = 'New Opportunity',
                                    CloseDate = System.today(),
                                    StageName = '1. Initiated Evaluation',
                                    Type ='New',
                                    RecordTypeId = NewRecordTypeId,
                                    AccountId = acct1.Id,
                                    Payment_Terms__c = 'Annual',
                                    Contract_Start_Date__c = System.today(),
                                    Contract_End_Date__c = System.today() + 364,
                                    Amount = 10000,
                                    OwnerId = userinfo.getuserid());



Insert newOpp;
OpportunityLineItem oli = new OpportunityLineItem(OpportunityId = newOpp.Id,
                                                  PricebookEntryId = pbEntry.Id,
                                                  Quantity = 1,
                                                  UnitPrice = pbEntry.UnitPrice
                                                  );
Insert oli; 
Test.startTest();
newOpp.StageName = 'Contract Signed';
Update newOpp;    
Test.stopTest();
Opportunity OppRecord = [select StageName from Opportunity where Id =: newOpp.Id];

System.assertEquals('Contract Signed',OppRecord.StageName);
}

}