Parent queueable class:
public with sharing class Qapex implements Queueable{
    private list<Account> L;
        public Qapex(List<Account> a) {
            this.L= a;
        }
    public void execute(QueueableContext q){
        list<Account> LA= new list<Account>();
        for(account f: this.L){
            account acc= new account();
            acc.Name= 'queable name' + f.Name;
            LA.add(acc);
        }
        INSERT LA;
        system.enqueueJob(new Qapex1());
    }
    }

Child queueable class: 
public with sharing class Qapex1 implements Queueable{
    public void execute(QueueableContext QC){
        account ac = new account(name= 'new name');
        insert ac;
    }
   
}

Trigger:
trigger QapexTgr on Account (before insert) {
 if(trigger.isBefore){
     if(trigger.isInsert){
         system.enqueueJob(new Qapex(trigger.new));
     }
 }
}

Parent queuable test class:
@istest
public with sharing class QapexTCLS {
    public testmethod static void QapexTCLSM(){
            Account a= new Account();
            a.Name= 'test account';
        
        insert a;
        test.startTest();
        system.enqueueJob(new Qapex(trigger.new));
        test.stopTest();

        account acc= [SELECT ID, name FROM account WHERE ID = : A.ID];
        system.assertEquals('test account', a.name);
    }

}

Child queueable test class:
@istest
public class Qapex1TCls {
   @istest
    public static void Qapex1TClsM() {
    account a= new account(name= 'qapextestname');
    insert a;

    test.startTest();
    system.enqueueJob(new Qapex1());
    test.stopTest();

    account ac= [select id, name from account where id= : a.id];
    system.assertEquals('qapextestname', a.name);
    }
}

ERROR:
QapexTCLS.QapexTCLSM  Fail     System.AsyncException: Maximum stack depth has been reached.  
                               Class.Qapex.execute: line 14, column 1   

This class name's value is invalid: Qapex1TCls. Provide the name of an Apex class that has test methods.