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

Imported from https://github.com/CleanCocoa/DeclarativeTextKit/issues/6

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E.g. `NSTextViewBuffer`, `MutableStringBuffer`, `Undoable` 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:

```swift
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:

```swift
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