add tests for Object.assign

test/built-ins/Object/assign/ObjectOverride-sameproperty.js

test/built-ins/Object/assign/OnlyOneArgument.js

test/built-ins/Object/assign/Override-notstringtarget.js

test/built-ins/Object/assign/Override.js

test/built-ins/Object/assign/Source-Null-Undefined.js

test/built-ins/Object/assign/Source-Number-Boolen-Symbol.js

test/built-ins/Object/assign/Source-String.js

test/built-ins/Object/assign/Target-Boolean.js

test/built-ins/Object/assign/Target-Null.js

test/built-ins/Object/assign/Target-Number.js

test/built-ins/Object/assign/Target-Object.js

test/built-ins/Object/assign/Target-String.js

test/built-ins/Object/assign/Target-Symbol.js

test/built-ins/Object/assign/Target-Undefined.js

test/built-ins/Object/assign/assign-descriptor.js

test/built-ins/Object/assign/assign-length.js