[3.15] improve json test coverage (GH-154123) (GH-154134)

Add test that covers the except block on line 106 of decoder.py
(cherry picked from commit ee1e48e)

Co-authored-by: David <slavicek.david29@gmail.com>

Author: miss-islington

Lib/test/test_json/test_decode.py

@@ -149,6 +149,10 @@ def test_negative_index(self):
         d = self.json.JSONDecoder()
         self.assertRaises(ValueError, d.raw_decode, 'a'*42, -50000)
 
+    def test_unterminated_string(self):
+        d = self.json.JSONDecoder()
+        self.assertRaises(self.JSONDecodeError, d.raw_decode, '"\\')
+
     def test_limit_int(self):
         maxdigits = 5000
         with support.adjust_int_max_str_digits(maxdigits):