Add unit tests for FlowId class, and update pom.xml to use guava-testlib library.

- Changed toString() method to return the value in HEX string.
- Added the guava-testlib library to pom.xml.
- Added unit tests for FlowId class.

Change-Id: I29c4d11a1737c9fac6a62e5541c5a98add46c295
diff --git a/pom.xml b/pom.xml
index a0bca46..24216b8 100644
--- a/pom.xml
+++ b/pom.xml
@@ -620,6 +620,12 @@
       <version>17.0</version>
     </dependency>
     <dependency>
+      <groupId>com.google.guava</groupId>
+      <artifactId>guava-testlib</artifactId>
+      <version>17.0</version>
+      <scope>test</scope>
+    </dependency>
+    <dependency>
       <groupId>commons-configuration</groupId>
       <artifactId>commons-configuration</artifactId>
       <version>1.10</version>