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

test/built-ins/String/15.5.5.5.2-1-1.js

test/built-ins/String/15.5.5.5.2-1-2.js

test/built-ins/String/15.5.5.5.2-3-1.js

test/built-ins/String/15.5.5.5.2-3-2.js

test/built-ins/String/15.5.5.5.2-3-3.js

test/built-ins/String/15.5.5.5.2-3-4.js

test/built-ins/String/15.5.5.5.2-3-5.js

test/built-ins/String/15.5.5.5.2-3-6.js

test/built-ins/String/15.5.5.5.2-3-7.js

test/built-ins/String/15.5.5.5.2-3-8.js

test/built-ins/String/15.5.5.5.2-7-1.js

test/built-ins/String/15.5.5.5.2-7-2.js

test/built-ins/String/15.5.5.5.2-7-3.js

test/built-ins/String/15.5.5.5.2-7-4.js

...ring/prototype/endsWith/String.prototype.endsWith_Fail.js

...ng/prototype/endsWith/String.prototype.endsWith_Fail_2.js

...g/prototype/endsWith/String.prototype.endsWith_Success.js

...prototype/endsWith/String.prototype.endsWith_Success_2.js

...prototype/endsWith/String.prototype.endsWith_Success_3.js

...prototype/endsWith/String.prototype.endsWith_Success_4.js