Add test() helper

[mrazauskas]

Playground link

Just to draw your attention, the first example in readme is not working. I have added playground link. It is rather obvious that test() is not imported.

Just an idea: perhaps expect-type could have its own test() helper?

[mmkal]

Thanks! That example assumes a globally available test value - it's from the jest days, but we should simplify so you can copy the example directly and have it work.