[Material3] Add RunnerTests target for macOS (#1670)

In https://github.com/flutter/samples/pull/1615 updates were applied to
the Material 3 demo that included the updated macOS Podfile from the
macOS app template. However, the app template itself had recently been
updated to refer to a new target (RunnerTests) in the default Xcode
project template that allows for authoring tests of the bits of the
Flutter application implemented in Swift. This adds the missing
RunnerTests bits to the Xcode project file by copying those over from
the app template.

material_3_demo/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.
+  }
+
+}