Fix offline test execution problem regarding maven not finding `org.a…

…pache.maven.surefire:surefire-junit4:jar:2.20.1` and `dummy:dummy:jar:1.0` dependencies while offline

Apparently the `surefire-junit4` dependency needs to be listed explicitly as the plugin's dependency, so that `mvn dependency:go-offline` could see it needs to be downloaded too.

There was a nice discussion over here:
  googleapis/google-cloud-java#3819
  googleapis/google-cloud-java#3820

pom.xml

@@ -181,6 +181,13 @@
             <exclude>**/PackageTest.java</exclude>
           </excludes>
         </configuration>
+        <dependencies>
+          <dependency>
+            <groupId>org.apache.maven.surefire</groupId>
+            <artifactId>surefire-junit4</artifactId>
+            <version>2.20.1</version>
+          </dependency>
+        </dependencies>
       </plugin>
 
       <plugin>