Add basic integration test

Summary:
Just a quick startup test that can save you the time of manually
opening the app.

I don't have any plans of hooking this up in Circle, but it's
still kinda useful to have.

Reviewed By: jknoxville

Differential Revision: D12901590

fbshipit-source-id: a16a1814574d9f27770dcf98c6b238517c589930

android/sample/src/androidTest/java/com/facebook/flipper/sample/MainActivityTest.java

@@ -0,0 +1,31 @@
+/*
+ *  Copyright (c) 2018-present, Facebook, Inc.
+ *
+ *  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.flipper.sample;
+
+import androidx.test.filters.LargeTest;
+import androidx.test.rule.ActivityTestRule;
+import androidx.test.runner.AndroidJUnit4;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+@LargeTest
+@RunWith(AndroidJUnit4.class)
+public class MainActivityTest {
+
+  @Rule
+  public final ActivityTestRule<MainActivity> mTestRule =
+      new ActivityTestRule<>(MainActivity.class);
+
+  @Test
+  public void activityStarts() {
+    final MainActivity activity = mTestRule.getActivity();
+
+    assert activity != null;
+  }
+}