RN: Copyright Header for `ReactInstanceManagerTest.java`

Summary: Missing when D12829677 landed.

Reviewed By: mdvacca

Differential Revision: D12903182

fbshipit-source-id: 1c142acb2a098b1118170f2f8ed8cf719fc14f90

ReactAndroid/src/androidTest/java/com/facebook/react/tests/core/ReactInstanceManagerTest.java

@@ -1,3 +1,8 @@
+//  Copyright (c) Facebook, Inc. and its affiliates.
+//
+// This source code is licensed under the MIT license found in the
+// LICENSE file in the root directory of this source tree.
+
 package com.facebook.react.tests.core;
 
 import android.app.Activity;