Add single squash merge e2e test

What does this MR do and why?

Add single squash merge e2e

When ' single squash merge ' is enabled, check the commit size after the user merges an MR.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.