Add syncify test helper

[chrisronline]

@cjcenizal recommended adding this from this discussion

This helper allows you to properly test async functions that throw exceptions.

Here is working code using the helper:

const fn = await syncify(() => asyncFn());
expect(fn).toThrow();

[cjcenizal]

LGTM! Just one request.

[cjcenizal]

Can we add your example code to this comment:

Example usage:
const fn = await syncify(() => asyncFn());
expect(fn).toThrow();

[cjcenizal]

Oh and can you add a note to the CHANGELOG.md?

[chrisronline]

Done!