In the test class, follow the below

1. Create Messaging.InboundEmail.

2. Create Messaging.InboundEnvelope.

3. Pass them to handleInboundEmail() method of Messaging.InboundEmailHandler class.

Sample Test Class Code:

Messaging.InboundEmail email = new Messaging.InboundEmail() ;
Messaging.InboundEnvelope env = new Messaging.InboundEnvelope();
email.subject = 'Test';
email.fromname = 'Test Test';
env.fromAddress = '[email protected]';
email.plainTextBody = 'Test';
CreateLeadInboundHandler emailProcess = new CreateLeadInboundHandler();
emailProcess.handleInboundEmail(
    email, env
);

here CreateLeadInboundHandler is a Email Service controller class.