Replace runTestCase with assert helpers [test/built-ins/JSON]

test/built-ins/JSON/15.12-0-1.js

test/built-ins/JSON/15.12-0-4.js

test/built-ins/JSON/parse/15.12.1.1-0-9.js

test/built-ins/JSON/parse/15.12.2-0-1.js

test/built-ins/JSON/parse/15.12.2-0-2.js

test/built-ins/JSON/parse/15.12.2-0-3.js

test/built-ins/JSON/stringify/15.12.3-0-1.js

test/built-ins/JSON/stringify/15.12.3-0-2.js

test/built-ins/JSON/stringify/15.12.3-0-3.js

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/15.12.3-11-26.js