Create tests for StringUtils#getStackTrace and StringUtils#getDifference

- Create tests
- Make StringUtils final with private constructor (utility class)

src/main/java/fr/xephi/authme/util/StringUtils.java

@@ -10,10 +10,14 @@ import java.io.StringWriter;
 /**
  * Utility class for String operations.
  */
-public class StringUtils {
+public final class StringUtils {
 
     public static final String newline = System.getProperty("line.separator");
 
+    private StringUtils() {
+        // Utility class
+    }
+
     /**
      * Get the difference of two strings.
      *