add macOS implementation

example/macos/RunnerTests/RunnerTests.swift

@@ -0,0 +1,12 @@
+import FlutterMacOS
+import Cocoa
+import XCTest
+
+class RunnerTests: XCTestCase {
+
+  func testExample() {
+    // If you add code to the Runner application, consider adding tests here.
+    // See https://developer.apple.com/documentation/xctest for more information about using XCTest.
+  }
+
+}