Add overloaded form of unit test declaration: test(name, fn)

[ry]

There are currently two ways to declare a unit test

// original form
test(function foo() { assert(true); });
// expanded form
test({ name: "foo", fn: () => { assert(true); });

It turns out that many of the tests in deno_std are using the expanded form. We could shorten this significantly by also allowing

test( "foo", () => { assert(true); });

Someone has suggested this before and noted that a certain existing test framework uses this form as well. (I can't find a reference.) So that's another reason to change.

Edit: reportedly Ava.js, Mocha and Jest all support this.

[sholladay]

I suggested it in #93.

More specifically, I think AVA's API should be adopted, for at least test definitions and assertions.