UI testing openURL and universal links via iMessage

iMessage.swift

import Foundation
import XCTest

//This is based on https://blog.branch.io/ui-testing-universal-links-in-xcode-9/
final class iMessage {

    static func launch() -> XCUIApplication {
        // Open iMessage App
        let messageApp = XCUIApplication(bundleIdentifier: "com.apple.MobileSMS")
        
        // Launch iMessage app
        messageApp.launch()
        
        // Wait some seconds for launch
        XCTAssertTrue(messageApp.waitForExistence(timeout: 10))
        
        // Continues "Whats new" if present
        let continueButton = messageApp.buttons["Continue"]
        if (continueButton.exists) {
            continueButton.tap()
        }
        
        // Removes New Messages Sheet on iOS 13
        let cancelButton = messageApp.navigationBars.buttons["Cancel"]
        if cancelButton.exists {
            cancelButton.tap()
        }
        
        // Return application handle
        return messageApp
    }

    static func open(URLString urlString: String, inMessageApp app: XCUIApplication) {
        XCTContext.runActivity(named: "Open URL \(urlString) in iMessage") { _ in
            // Find Simulator Message
            let kateBell = app.cells.staticTexts["Kate Bell"]
            XCTAssertTrue(kateBell.waitForExistence(timeout: 10))
            kateBell.tap()

            // Tap message field
            app.textFields["iMessage"].tap()
                                                                            
            // Continues "Swipe to Text" Sheet
            let continueButton = app.buttons["Continue"]
            if continueButton.exists {
                continueButton.tap()
            }

            // Enter the URL string
            app.typeText("Open Link:\n")
            app.typeText(urlString)

            // Simulate sending link
            app.buttons["sendButton"].tap()

            //Wait for Main App to finish launching
            sleep(2)

            // The first link on the page
            let messageBubble = app.cells.links["com.apple.messages.URLBalloonProvider"]
            XCTAssertTrue(messageBubble.waitForExistence(timeout: 10))
            messageBubble.tap()
        }
    }

    static func openFromPasteboard(inMessageApp app: XCUIApplication) {
        XCTContext.runActivity(named: "Open URL from Pasetboard in iMessage") { _ in
            // Find Simulator Message
            let kateBell = app.cells.staticTexts["Kate Bell"]
            XCTAssertTrue(kateBell.waitForExistence(timeout: 10))
            kateBell.tap()
            
            // Add dummy text to workaround issue with link-only bubbles in iOS 12.4+
            app.textFields["iMessage"].tap()
            app.typeText("Open Link:\n")
            
            // Tap message field to open paste menu
            app.textFields["iMessage"].tap()
            
            sleep(1)
            app.menuItems["Paste"].tap()
            
            // Simulate sending link
            sleep(1)
            app.buttons["sendButton"].tap()
            
            //Wait for Main App to finish launching
            sleep(3)
            
            // The first link on the page
            let messageBubble = app.cells.links["com.apple.messages.URLBalloonProvider"]
            XCTAssertTrue(messageBubble.waitForExistence(timeout: 10))
            messageBubble.tap()
        }
    }
}

UniversialLinkTest.swift

import XCTest

class UniversialLinktTest: XCTestCase {
    var app: XCUIApplication!

    override func setUp() {
        super.setUp()
        continueAfterFailure = false
        app = XCUIApplication()
        app.launch()
    }

    func testItDeepLinksIntoApp() {
        // Open App via iMessage
        let messageApp = iMessage.launch()
        iMessage.open(URLString: "https://mydomain.com/verify/12345", inMessageApp: messageApp)
        
        // Do test here...
        let isInVerifyScreen = app.staticTexts["12345"].waitForExistence(timeout: 10)
        XCTAssertTrue(isInVerifyScreen)

        // For reliability terminate iMessage after test
        messageApp.terminate()
    }
    
}

Yeah, I saw this thread just after I posted here. Seems we need to wait some more for a non-workaround (native) solution.