Add an experimental CustomTestReflectable protocol.

[grynspan]

This PR adds CustomTestReflectable as experimental SPI. This protocol can be used to customize the breakdown of a value shown when an expectation fails. For example, given this structure and test:

struct MyStruct {
  var x: Int
  func checkX(_ expected: Int) -> Bool { x == expected }
}

@Test func foo() {
  let value = MyStruct(x: 1234)
  #expect(value.checkX(6789))
}

A failure would currently be reported as:

<X>  Test "..." recorded an issue at [...]: Expectation failed: value.checkX(6789)
`->  value.checkX(6789) → false
`->    value → MyStruct(x: 1234)
`->      x → 1234

If we add a custom mirror to it, like so:

extension MyStruct: CustomTestReflectable {
  var customTestMirror: Mirror {
    Mirror(
      self,
      children: [
        (label: "x", value: x as Any),
        (label: "x is even?", value: x.isMultiple(of: 2)),
        (label: "x is positive?", value: x > 0),
      ]
    )
  }
}

Then the output becomes:

<X>  Test "..." recorded an issue at [...]: Expectation failed: value.checkX(6789)
`->  value.checkX(6789) → false
`->    value → MyStruct(x: 1234)
`->      x → 1234
`->      x is even? → true
`->      x is positive? → true

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.