Improve code coverage.

It's embarrassing having only 25% code coverage with literally just
Hello World. This should bump it up by subbing out stdout. See:
http://stackoverflow.com/a/2169336/101923

Author: tswast

test/src/main/java/com/google/cloud/samples/test/App.java

@@ -20,11 +20,12 @@
  * A hello world app to test the parent pom.xml.
  */
 public class App {
-  public static String greeting() {
+  public String greeting() {
     return "Hello World!";
   }
 
   public static void main(String[] args) {
-    System.out.println(App.greeting());
+    App app = new App();
+    System.out.println(app.greeting());
   }
 }

test/src/test/java/com/google/cloud/samples/test/AppTest.java

@@ -22,12 +22,21 @@
 import org.junit.runner.RunWith;
 import org.junit.runners.JUnit4;
 
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+
 /**
  * Unit tests for {@link App}.
  */
 @RunWith(JUnit4.class)
 public class AppTest {
-  @Test public void greeting_returnsHelloWorld() {
-    assertThat(App.greeting()).named("greeting").isEqualTo("Hello World!");
+  @Test public void main_printsHelloWorld() {
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    System.setOut(new PrintStream(out));
+
+    App.main(new String[0]);
+
+    String greeting = out.toString();
+    assertThat(greeting).named("greeting").contains("Hello World!");
   }
 }