How to write a test class for trigger having single email messaging

trigger sendEmailtoinitiator on Case (after update) {

List<Messaging.SingleEmailMessage> mails = new List<Messaging.SingleEmailMessage>();

Map<Id, Case> objMap = new Map<Id, Case>([ SELECT Id, Allocated_By__r.Email FROM Case WHERE Id IN : Trigger.new ]);

for (Case newCase : Trigger.new){

if((newCase.RCU_Stage__c == 'RCU Completion'|| newCase.RCU_Stage__c == 'RCU Clear' || newCase.RCU_Stage__c == 'RCU Negative')

&& newCase.RCU_completion_Status__c !=null && newCase.Completion_Submit__c == true ){

Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();

String[] toAddresses = new String[] {(objMap.get(newCase.Id).Allocated_By__r.Email)};

mail.setToAddresses(toAddresses);

EmailTemplate et = [select id,Subject,Body from EmailTemplate where developername='CaseStageEmailAlert'];

mail.setTemplateId(et.Id );

mail.setWhatId(newCase.Id);

mail.setSubject(et.Subject);

mail.setPlainTextBody(et.Body);

mail.setTreatBodiesAsTemplate(true);

//adding email to master list

mails.add(mail);

}

}

Messaging.sendEmail(mails);

}

Please help me out with this salesforce query.