VS Code and Git - How to managin code required for testing?

What is the recommended way to manage code changes that I have to do only for testing?

I am commenting out code and adding in code only for testing purposes. E.g. Prod script will download files from S3 bucket but for testing I only want to grab the files on my local system. Another example is disabling code that I don't want to run during testing such as deleting files after processing, or skipping different types of files.

I do manage different branches in git for features and development but I can't seem to figure out how to just make these changes in development and then not include them in a merge to master.

Thanks