difference_of_squares: Sync tests & add template

[ellnix]

I considered adding the tera_text_filters crate but decided on if statements to avoid adding more dependencies.

With a snake_case filter, the template could be written without the function variable.

P. S.: I didn't forget to run update-exercise there is just no diff on the test file.

[no important files changed]

[senekor]

I don't think getting rid of the if statement is worth adding a dependency. You could implement a simple version of such a filter yourself if you wanted to.

[ellnix]

I don't think getting rid of the if statement is worth adding a dependency. You could implement a simple version of such a filter yourself if you wanted to.

Agreed, one if statement is no big deal, if it comes up in another template I'll probably make a short naive snake_case. I'm glad I got your opinion on it first 😄