Create swift-testing.md for XCTest tutorial

Added a new article on testing Swift applications using XCTest.

Author: monodot

_articles/swift-testing.md

@@ -0,0 +1,21 @@
+---
+layout: page
+title: Swift Testing
+lede: "Testing Swift applications for iOS"
+---
+
+## XCTest
+
+### Verify an onboarding flow shows at first launch
+
+```swift
+@MainActor
+func testOnboardingShowsAtFirstLaunch() throws {
+    let app = XCUIApplication()
+    app.launchArguments = ["-\(UserDefaultsKeys.onboardingCompleted)", "NO"]
+    app.launch()
+    
+    XCTAssertTrue(app.staticTexts["Some Onboarding Screen Text"].waitForExistence(timeout: 5)) // Wait for the first onboarding screen to show
+    XCTAssertTrue(app.buttons["The Next Button"].exists)
+}
+```