Add tests for async functions

test/built-ins/AsyncFunction/AsyncFunction-construct.js

test/built-ins/AsyncFunction/AsyncFunction-is-extensible.js

test/built-ins/AsyncFunction/AsyncFunction-is-subclass.js

test/built-ins/AsyncFunction/AsyncFunction-length.js

test/built-ins/AsyncFunction/AsyncFunction-name.js

test/built-ins/AsyncFunction/AsyncFunction-prototype.js

test/built-ins/AsyncFunction/AsyncFunction.js

...ins/AsyncFunction/AsyncFunctionPrototype-is-extensible.js

...ilt-ins/AsyncFunction/AsyncFunctionPrototype-prototype.js

...ilt-ins/AsyncFunction/AsyncFunctionPrototype-to-string.js

test/built-ins/AsyncFunction/instance-construct.js

test/built-ins/AsyncFunction/instance-has-name.js

test/built-ins/AsyncFunction/instance-length.js

test/built-ins/AsyncFunction/instance-prototype-property.js

test/built-ins/AsyncFunction/is-not-a-global.js

test/built-ins/Function/prototype/toString/AsyncFunction.js

...Function/prototype/toString/async-function-declaration.js

.../Function/prototype/toString/async-function-expression.js

test/built-ins/Function/prototype/toString/async-method.js

...expressions/async-arrow-function/arrow-returns-promise.js