Fix bugs in JSON test

* test/src/json-tests.el (json-serialize/invalid-unicode): Fix two
bugs that canceled each other out.  "a\xCCb" is actually a valid
Unicode string because the hexadecimal character escape isn't
terminated by the "b".  But this was masked by an incorrect
closing parentheses, causing an unrelated error.

test/src/json-tests.el

@@ -89,7 +89,7 @@
   (should-error (json-serialize ["a\uDBBBb"]) :type 'json-out-of-memory)
   (should-error (json-serialize (vector (string ?a #x110000 ?b)))
                 :type 'json-out-of-memory)
-  (should-error (json-serialize ["a\xCCb"] :type 'json-out-of-memory)))
+  (should-error (json-serialize ["u\xCCv"]) :type 'json-out-of-memory))
 
 (ert-deftest json-parse-string/null ()
   (skip-unless (fboundp 'json-parse-string))