python: Add tests for trying to use an invalid Inferior object
    
    This patch adds tests for trying to use property or methods on a
    gdb.Inferior object that represents an inferior that does not exist
    anymore.  We expect an exception to be thrown.
    
    gdb/testsuite/ChangeLog:
    
    	* gdb.python/py-inferior.exp: Test using an invalid gdb.Inferior
    	object.