`t.throwsAsync()` and `t.notThrowsAsync()` should be awaited on #274

When you use the t.throwsAsync() and t.notThrowsAsync() assertion, you must await the promise they return. If the test function completes before the assertions do the test will fail.

We should add a rule to ensure users invoke these assertions with the await keyword. I'm not sure whether it should be recommended — you could assign the promise to a variable and await it later, but I don't think that's typical when you write tests.

See also the discussion in https://github.com/avajs/ava/issues/2245.

@sindresorhus?

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