Hi Krishnamurthy,
 
    You need to separate the code to send email from your trigger to a util class. Please see code below. Hope this helps.


/**util class to send email**/

public class MailerUtil
{
    public static void sendMailMessage(List <ContentVersion> conVer) 
    {
        List<Messaging.SingleEmailMessage> allMessages = new List<Messaging.SingleEmailMessage>(); 
        
            List<Invoice_Payment__c> invoiceList = [Select Id, Name
                                            From Invoice_Payment__c 
                                           where Id IN (SELECT FirstPublishLocationId FROM ContentVersion WHERE Id IN :conVer)];
        
            for(ContentVersion cv : conVer)
            {

                Messaging.EmailFileAttachment attachment = new Messaging.EmailFileAttachment(); 
                attachment.setBody(Blob.valueof(cv.ContentBodyId));
                attachment.setFileName(cv.Title + '.' + cv.FileType); 
                
                Messaging.SingleEmailMessage message = new Messaging.SingleEmailMessage(); 
                
                message.setToAddresses(new String[] { 'andre.nobre@cleantechsolar.com' ,'rupesh.baker@cleantechsolar.com'});
                message.setccAddresses(new String[] { 'krishnamurthy.ka@cleantechsolar.com' ,'divina.minoza@cleantechsolar.com'});
                
                //Subject with the retrieved Invoice Number
                message.subject = 
                String.format('Your Invoice having Invoice # {0} is due for payment', 
                        new String[]{invoiceList[0].Name}); 
                
                //Email body containing Name with the retrieved invoice
                message.plainTextBody = 
                String.format('Your invoice for {0} with the file name {1} is attached alongwith.', 
                new String[]{invoiceList[0].Name, cv.Title});
                        
                message.setFileAttachments(new Messaging.EmailFileAttachment[] {attachment}); 
                allMessages.add(message); 
            } 
        
        Messaging.sendEmail(allMessages); 
    }
   
}



/**trigger**/
trigger EmailTrigger on ContentVersion (after insert){
if(Trigger.isAfter && Trigger.isInsert){
    MailerUtil.sendMailMessage(trigger.new);
}
}

/**test class**/
@istest
public class MailerUtilTest {
    public static testmethod void testvalidate(){
        //insert invoice payment record
        //insert a record of ContentVersion object -- this will fire the trigger and the code to send email 
        //will be executed
    }
}    
}

Regards,