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

test/built-ins/Date/15.9.1.15-1.js

test/built-ins/Date/now/15.9.4.4-0-3.js

test/built-ins/Date/prototype/toISOString/15.9.5.43-0-11.js

test/built-ins/Date/prototype/toISOString/15.9.5.43-0-12.js

test/built-ins/Date/prototype/toISOString/15.9.5.43-0-4.js

test/built-ins/Date/prototype/toISOString/15.9.5.43-0-5.js

test/built-ins/Date/prototype/toJSON/15.9.5.44-0-1.js

test/built-ins/Date/prototype/toJSON/15.9.5.44-0-2.js