Suppose you have a class like this for recieving email
/**
* Email services are automated processes that use Apex classes
* to process the contents, headers, and attachments of inbound
* email.
create account from email subject
create contacts from email cc
create text and binary attachments if any

*/
global class EmailDemoReceive implements Messaging.Inboundemailhandler {

global Messaging.Inboundemailresult handleInboundemail(Messaging.Inboundemail email, Messaging.Inboundenvelope env ){
Account account;
Messaging.Inboundemailresult result = new Messaging.Inboundemailresult();
try{

if([Select count() from Account where Name =:email.subject ]==0){
account = new Account();
account.Name = email.subject;
insert account;
}else {
account=[select id from account where name=:email.subject ];
}

// Convert cc'd addresses to contacts
for (String address : email.ccAddresses) {
Contact contact = new Contact();
Matcher matcher = Pattern.compile('<.+>').matcher(address);

// Parse addresses to names and emails
if (matcher.find()) {
String[] nameParts = address.split('[ ]*<.+>')[0].replace('"', '').split('[ ]+');

contact.FirstName = nameParts.size() > 1 ? nameParts[0] : '';
contact.LastName = nameParts.size() > 1 ? nameParts[nameParts.size()-1] : nameParts[0];
contact.Email = matcher.group().replaceAll('[<>]', '');
} else {
contact.LastName = address;
contact.Email = address;
}

// Add if new
if ([select count() from Contact where Email = :contact.Email] == 0) {
contact.AccountId = account.Id;
insert contact;
}
}

for(Messaging.Inboundemail.Binaryattachment Battachment : email.binaryAttachments){
Attachment a = new attachment();
a.Name= battachment.fileName;
a.body= battachment.body;
a.parentID= account.id;
try{
insert a;}
catch(DMLException e){
system.debug(e.getMessage());
}
}

for(Messaging.Inboundemail.Textattachment tAttachment : email.textAttachments) {
Attachment a = new Attachment();
a.name= tAttachment.fileName;
a.body= blob.valueOf(tAttachment.body);
a.parentID= account.id;
insert a;
}

result.success= true;
}
catch(Exception e){
result.success=false;
result.Message='oops, failed again';

}

return result;
}

}



Unit class for testing this would be
 

@IsTest
private class EmailDemoReceiveHandlerTests {

       // Create a new email and envelope object
       Messaging.InboundEmail email  = new Messaging.InboundEmail();
       Messaging.InboundEnvelope env = new Messaging.InboundEnvelope();
      
       // Set up your data if you need to
      
       // Create the email body
       email.plainTextBody = 'This should become a note';
       email.fromAddress ='test@test.com';
       String contactEmail = 'jsmith@salesforce.com';
       email.ccAddresses = new String[] {'Jon Smith <' + contactEmail + '>'};
       email.subject = 'Dummy Account Name 123';
      
       EmailDemoReceive edr = new EmailDemoReceive();
      
       Test.startTest();
       Messaging.InboundEmailResult result = edr.handleInboundEmail(email, env);
       Test.stopTest();
      
       System.assert (result.success, 'InboundEmailResult returned a failure message');
      
       Account [] accDb = [select ID from Account where name=:email.subject];
       System.assertEquals (1, accDb.size(),'Account was not inserted');
       Contact [] cDb = [select firstname,lastname from Contact where email=:contactEmail];
       System.assertEquals (1, cDb.size(),'Contact was not inserted!');
       Contact c = CDb[0];
       System.assertEquals ('Jon', c.firstName);
       System.assertEquals ('Smith', c.LastName);
       Note [] nDb = [select body from Note where ParentID=:accDb[0].id];
       System.assertEquals (1,nDb.size(), 'A note should have been attached');
       System.assertEquals (email.plainTextBody, nDb[0].body);
      
   }
}