Simplify and improve control character escaping tests

test/built-ins/JSON/stringify/15.12.3-11-16.js

test/built-ins/JSON/stringify/15.12.3-11-17.js

test/built-ins/JSON/stringify/15.12.3-11-18.js

test/built-ins/JSON/stringify/15.12.3-11-19.js

test/built-ins/JSON/stringify/15.12.3-11-20.js

test/built-ins/JSON/stringify/15.12.3-11-21.js

test/built-ins/JSON/stringify/15.12.3-11-22.js

test/built-ins/JSON/stringify/15.12.3-11-23.js

test/built-ins/JSON/stringify/15.12.3-11-24.js

test/built-ins/JSON/stringify/15.12.3-11-25.js

test/built-ins/JSON/stringify/string-escape-ascii.js