Extended tests coverage for SE instance

What does this MR do and why?

Just improving the test coverage for the Source Editor instance to make sure we pass correct arguments and in correct order down to the extension's methods. Also make sure that the this context is correct when extension's method is invoked.

Screenshots or screen recordings

No visual or user-facing changes

How to set up and validate locally

Just run the spec

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.

Related to #292943 (closed)