How to test client app without creating a server?

Is there anyway to create a server response manually to check whether my client side is working correctly to server responses? I want to do this without writing the server part myself.
What I specifically want is to test a server-sent event handling part.

There are multiple extensions for Firefox and Chrome allowing you to edit the response.

Firefox:

• Modify HTTP Response (only allows to modify the response body)
• Modify Response Headers

Chrome:

• ModHeader
• Tamper Chrome (extension) (new version: Tamper Dev)
• Requestly