Add JSONString test class.

This set of tests demonstrates what happens when JSONString returns various results from its toJSONString() method. Tests for null returns and exceptions thrown. Also tests what happens for non-JSONString objects. The intent is to cover JSONObject's valueToString() and writeValue() methods.

src/test/java/org/json/junit/JunitTestSuite.java

@@ -15,7 +15,8 @@ import org.junit.runners.Suite;
    JSONObjectTest.class,
    JSONArrayTest.class,
    EnumTest.class,
-   JSONPointerTest.class
+   JSONPointerTest.class,
+   JSONStringTest.class
 })
-public class JunitTestSuite {   
-}  
+public class JunitTestSuite {
+}