restructured the add to app samples (#698)

add_to_app/plugin/ios_using_plugin/IOSUsingPluginTests/IOSUsingPluginTests.swift

@@ -0,0 +1,30 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import XCTest
+@testable import IOSUsingPlugin
+
+class IOSUsingPluginTests: XCTestCase {
+
+    override func setUp() {
+        // Put setup code here. This method is called before the invocation of each test method in the class.
+    }
+
+    override func tearDown() {
+        // Put teardown code here. This method is called after the invocation of each test method in the class.
+    }
+
+    func testExample() {
+        // This is an example of a functional test case.
+        // Use XCTAssert and related functions to verify your tests produce the correct results.
+    }
+
+    func testPerformanceExample() {
+        // This is an example of a performance test case.
+        self.measure {
+            // Put the code you want to measure the time of here.
+        }
+    }
+
+}