Document purpose of test-jar and testlib dependencies (#4164)

Author: chingor13

java-pubsub/google-cloud-pubsub/pom.xml

@@ -51,12 +51,8 @@
       <artifactId>auto-value</artifactId>
       <scope>compile</scope>
     </dependency>
-    <dependency>
-      <groupId>${project.groupId}</groupId>
-      <artifactId>google-cloud-core</artifactId>
-      <type>test-jar</type>
-      <scope>test</scope>
-    </dependency>
+
+    <!-- Test dependencies -->
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
@@ -82,6 +78,7 @@
       <artifactId>grpc-google-iam-v1</artifactId>
       <scope>test</scope>
     </dependency>
+    <!-- Need testing utility classes for generated gRPC clients tests -->
     <dependency>
       <groupId>com.google.api</groupId>
       <artifactId>gax-grpc</artifactId>