Improve test coverage for Symbol (#645)

test/built-ins/Symbol/for/create-value.js

test/built-ins/Symbol/for/prop-desc.js

test/built-ins/Symbol/for/retrieve-value.js

test/built-ins/Symbol/for/to-string-err.js

test/built-ins/Symbol/keyFor/arg-non-symbol.js

test/built-ins/Symbol/keyFor/arg-symbol-registry-hit.js

test/built-ins/Symbol/keyFor/arg-symbol-registry-miss.js

test/built-ins/Symbol/keyFor/prop-desc.js

test/built-ins/Symbol/prototype/constructor.js

test/built-ins/Symbol/prototype/toString/prop-desc.js

test/built-ins/Symbol/prototype/valueOf/prop-desc.js

test/built-ins/Symbol/prototype/valueOf/this-val-non-obj.js

...t-ins/Symbol/prototype/valueOf/this-val-obj-non-symbol.js

...built-ins/Symbol/prototype/valueOf/this-val-obj-symbol.js

test/built-ins/Symbol/prototype/valueOf/this-val-symbol.js