Handle attached screenshots

[idrougge]

Since a while, XCUITest can take screenshots by itself and "attach them" to the .xcresult file for inspection in Xcode. I use that to snapshot the state when a UI test fails:

    override func recordFailure(withDescription description: String, inFile filePath: String, atLine lineNumber: Int, expected: Bool) {
        let screenshot = app.screenshot()
        let screenshotAttachment = XCTAttachment(screenshot: screenshot)
        screenshotAttachment.name = "\(name)-screenshot"
        screenshotAttachment.lifetime = .deleteOnSuccess
        add(screenshotAttachment)
        super.recordFailure(withDescription: description, inFile: filePath, atLine: lineNumber, expected: expected)
    }

This seems to be ignored by xcpretty, even when specifying the --screenshots command line argument.