Unit-test paredit.backspace where start != end

[pbwolf]

What has changed?

A new unit test checks the first logic branch in paredit.backspce: a non-empty selection.

I suppose paredit.backspace works properly in practice, but the new test failed. The test's simulator (StringDocument) did not attend to the non-empty selection case and always removed one character. I gather EditableDocument.backspace is used only by paredit.backspace(?), so harmonizing it with the actual editor should be all right. Paredit backspace uses doc.backspace twice, under distinct circumstances, but the second usage was already trodden by unit tests.

Fixes #2681

My Calva PR Checklist

I have:

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Ping @PEZ, @bpringe, @corasaurus-hex, @Cyrik

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[PEZ]

Thanks! 🙏