im trying to do the test class for this code

public class Claseprogramadausuario {
    
public static void obtenerinfo(){

List<User> nlist = [SELECT Id, Username, FirstName, LastName, EmployeeNumber, UserRole.Name, Profile.Name, CreatedDate, LastModifiedDate, LastLoginDate, isActive FROM User WHERE Profile.Name != 'SelfService'];
string header = 'Id, Username , FirstName, LastName, EmployeeNumber, UserRole.Name,  Profile.Name, Created Date, LastModifiedDate, LastLoginDate, isActive \n';
string finalstr = header ;
for(User u: nlist)
{

       string recordString = u.id+','+u.Username +',' + u.FirstName +','+ u.LastName +','+ u.EmployeeNumber+','+ u.UserRole.name +','+ u.profile.name +','+ u.CreatedDate +','+ u.LastModifiedDate +','+ u.LastLoginDate +','+ u.IsActive +'\n';

       finalstr = finalstr +recordString;
}

Messaging.EmailFileAttachment csvAttc = new Messaging.EmailFileAttachment();
blob csvBlob = Blob.valueOf(finalstr);
string csvname= 'Userinfo.csv';
csvAttc.setFileName(csvname);
csvAttc.setBody(csvBlob);
Messaging.SingleEmailMessage correo = new Messaging.SingleEmailMessage();
correo.toAddresses = new String[] {'javierandoni@gmail.com', 'dlramirez@izzi.mx'};
correo.subject = 'Clase programada test';
correo.plainTextBody = 'Clase programada test';
Messaging.SingleEmailMessage[] correos =   new List<Messaging.SingleEmailMessage> {correo};
correo.setPlainTextBody('Account CSV ');
correo.setFileAttachments(new Messaging.EmailFileAttachment[]{csvAttc});
Messaging.SendEmailResult[] results = Messaging.sendEmail(correos);

if (results[0].success) 
{
System.debug('The email was sent successfully.');
} else 
{
System.debug('The email failed to send: ' + results[0].errors[0].message);
}
}
}

And the test class im trying to do is this

// Test Class
@IsTest
Public class ClaseprogramadausuarioTest {
    @isTest static void testenviomail(){
        string pri= Claseprogramadausuario.obtenerinfo();
        System.assertEquals('The email was sent successfully.', pri);
    }
}

but i get this message: Illegal assignment from void to String.