test AsyncGeneratorFunction's basic properties

[thefourtheye]

Included basic tests for properties of AsyncGeneratorFunction to begin with.

cc @benjamingr

[benjamingr]

Great start, I'll work on it this Friday!

[caitp]

Based on @bterlson's earlier work on tests for async functions, I think built-ins/AsyncGeneratorFunction/ is a good place to put tests on properties of AsyncGeneratorFunction instances.

How about something like this?

// https://tc39.github.io/proposal-async-iteration/#sec-asyncgenerator-definitions-instantiatefunctionobject
async function* fn() { };
var AsyncGeneratorFunction = fn.constructor;
assert.sameValue(Object.getPrototypeOf(fn.prototype), AsyncGeneratorFunction.prototype.prototype,
	         "fn.prototype should have a [[Prototype]] of %AsyncGeneratorPrototype%");
verifyWritable(fn, "prototype");
verifyNotEnumerable(fn, "prototype");
verifyNotConfigurable(fn, "prototype");

Brian's work has some other good examples of useful tests, like verifying the behaviour of CreateDynamicFunction + async generators

[benjamingr]

Going to copy this to the new PR.

[benjamingr]

Copied what I could and rewrote the rest in the new format.