Fix and Test String.equals(Object) Implementation in JPF

[shreyasCs012]

This pull request addresses a bug in the Java Pathfinder(JPF) related to implementing the 'String. equals(Object)' method. I have made the following changes :

Test Addition :

I added a test to check if the equals method works if a non-String object was passed as an argument but the previous implementation of the equals method did not check this condition which was revealed via test.

Bug Fix :

1. argRef Handling:

   • The argRef variable which is one of the parameters passed in 'equals__Ljava_lang_Object_2__Z '. The purpose of the argRef
     is a reference to the symbolic object in the jpf heap but it does not check if the object is a String object before comparing
     the objects(concrete).

2. Updates :

   • The method now uses env.isInstanceOf(argRef, "java.lang.String") which checks if the object is a String before it checks the
     equality of the different objects.

[cyrille-artho]

Thanks!