Added Unit tests for StringUtil class

I have added unit tests for StringUtil class whichc has not yet been covered with the tests, so the coverage of this file
went from 0% to 85% (line coverage). I still have to add additional tests for "splitValues" method, which has not et been tested
because I do not want to exceed 1k LOC.

Signed-off-by: Leonardo Gaube <leonardo.gaube@comtrade.com>

commons/src/test/java/org/eclipse/kapua/commons/util/KapuaErrorCodesTest.java

@@ -0,0 +1,30 @@
+/*******************************************************************************
+ * Copyright (c) 2020 Eurotech and/or its affiliates and others
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Public License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/legal/epl-v10.html
+ *
+ * Contributors:
+ *     Eurotech - initial API and implementation
+ *******************************************************************************/
+package org.eclipse.kapua.commons.util;
+
+import org.eclipse.kapua.qa.markers.junit.JUnitTests;
+import org.junit.Assert;
+import org.junit.Test;
+import org.junit.experimental.categories.Category;
+
+@Category(JUnitTests.class)
+public class KapuaErrorCodesTest extends Assert {
+
+    @Test
+    public void testIdGenerationError() {
+        try {
+            assertNotNull(KapuaCommonsErrorCodes.valueOf("ID_GENERATION_ERROR"));
+        } catch (Exception ex) {
+            fail("ID_GENERATION_ERROR Enum does not exist");
+        }
+    }
+}