Allow docstrings on tests

[jariji]

I like to put docstrings on my tests so I know what they are testing.

"abs returns a nonnegative value."
@check function abs_nonneg(x=Floats())
  zero(x) <= abs(x)
end

ERROR: cannot document the following expression

[Seelengrab]

I don't think there's anything I can do to make that syntax work; you just can't put a docstring on a begin block expression (which is what's returned by @check) like that. However, since abs_nonneg is a function like any other, you can document it with @doc, or by documenting a function prototype:

"abs returns a nonnegative value."
function abs_nonneg end