Add shared testing harness for all Buffer-conforming types we ship

[DivineDominion]

Imported from CleanCocoa/DeclarativeTextKit#6

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E.g. NSTextViewBuffer, MutableStringBuffer, Undoable<Buffer> should all behave the same.

I went with code duplication for each of these so far, but that's getting a bit tedious. I notice that I'm feeling a resistance towards adding new shared Buffer features or changing the API because I need to touch 3+ test suites.

A shared suite would be cool. As a matter of fact, I have never used something like that with XCTest, though.

A naive approach would be to merge all test files into 1, and then add a helper that transforms this:

    func testDelete() throws {
        let buffer = MutableStringBuffer("Hello: world!")
        buffer.insertionLocation = length(of: "Hello: wor")

        assertBufferState(buffer, "Hello: wor{^}ld!")

        try buffer.delete(in: .init(location: 0, length: 4))
        assertBufferState(buffer, "o: wor{^}ld!")

        try buffer.delete(in: .init(location: 0, length: 4))
        assertBufferState(buffer, "or{^}ld!")

        try buffer.delete(in: .init(location: 0, length: 4))
        assertBufferState(buffer, "{^}!")
    }

... into something like this:

    func testDelete() throws {
        let originalContent = "Hello: world!"
        foreach buffer in [
            MutableStringBuffer(originalContent), 
            textView(originalContent),
            Undoable(MutableStringBuffer(originalContent)),
        ] { 
          buffer.insertionLocation = length(of: "Hello: wor")
  
          assertBufferState(buffer, "Hello: wor{^}ld!")
  
          try buffer.delete(in: .init(location: 0, length: 4))
          assertBufferState(buffer, "o: wor{^}ld!")
  
          try buffer.delete(in: .init(location: 0, length: 4))
          assertBufferState(buffer, "or{^}ld!")
  
          try buffer.delete(in: .init(location: 0, length: 4))
          assertBufferState(buffer, "{^}!")
        }
    }

• The loop should be in a block-based test helper for reuse
• Test failure messages need to mention which buffer type failed to meet the condition