Fix failures in coverage tests

Fix some failures when cobertura is run.  The coverage tool puts
extra fields into immutable objects and breaks the immutable contract.
The fix is to ignore the "__cobertura" fields.

Change-Id: Iff4d9edf5d7d150b6379dbc62de2984f8412ea24
diff --git a/src/test/java/net/onrc/onos/core/util/ImmutableClassChecker.java b/src/test/java/net/onrc/onos/core/util/ImmutableClassChecker.java
index 6002c58..12c9b0f 100644
--- a/src/test/java/net/onrc/onos/core/util/ImmutableClassChecker.java
+++ b/src/test/java/net/onrc/onos/core/util/ImmutableClassChecker.java
@@ -37,6 +37,10 @@
 
         // class must have only final and private data members
         for (final Field field : clazz.getDeclaredFields()) {
+            if (field.getName().startsWith("__cobertura")) {
+                //  cobertura sticks these fields into classes - ignore them
+                continue;
+            }
             if (!Modifier.isFinal(field.getModifiers())) {
                 failureReason = "a field named '" + field.getName() +
                                 "' that is not final";