Unit-test paredit backspace case where start != end

[pbwolf]

I did not find a unit test for the first logic branch in paredit.backspace, covering the case where start != end.

[bpringe]

@PEZ Any idea why this issue wasn't closed when the PR was merged? I noticed we no longer have the todo item in the PR template for adding a "fixes" note to a commit message, and I'm guessing because that's supposed to no longer be necessary. The fixes syntax is in the PR description, and I'm guessing it was there prior to merge. 🤔