@isTest
public static void testNewCaseInsert(){

    Id user = [SELECT id FROM User WHERE FirstName = 'TestUser' LIMIT 1].id;

    Id myPlusRTypeId    = [SELECT id FROM RecordType WHERE developerName = 'Test_Plus' LIMIT 1].id;

    Id contactId = [SELECT id FROM Contact WHERE FirstName = 'Contact' LIMIT 1].id;

    Case caseMPlus        = new Case(

        RecordTypeId         =  myPlusRTypeId,
        OwnerId              =  user,
        Status               = 'New',
        Priority             = 'Medium',
        ContactId            =  contactId,
        Origin               = 'Email',
        Subject              = 'Subject Mail Plus',
        Description          = 'watermelons',
     );

    test.setMock(HttpCalloutMock.class, new MockResponse(caseMPlus.id));//I can't pass case Id without inserting it first

    test.startTest();
        insert caseMPlus;
    test.stopTest();

   }

@isTest
public class LimitlessMockResponse implements HttpCalloutMock {

protected Id caseId;

public LimitlessMockResponse(Id caseId){

    this.caseId = caseId;

}


public HTTPResponse respond(HTTPRequest req) {

    System.assertEquals('callout:Limitless', req.getEndpoint());
    System.assertEquals('POST', req.getMethod());

    HttpResponse res = new HttpResponse();
    res.setHeader('Content-Type', 'application/json');
    //trying to pass the caseId into the body to assert against the response
    res.setBody('{"count": 1,"success": 1,"failed": 0,"messages": [{"tenantMsgId":'+'"'+caseId+'","rejectReasonCode": "no group found","processCode": "ACC"}]}');
    res.setStatusCode(200);
    return res;
}