Unify expect! and expect_file! types

[cpud36]

I want to use check pattern, but allow the user to select, whether to use inline expect! macro and the outline expect_file! macro.
But these return different types: Expect, and ExpectFile respectively. Would you mind merging this into a common api?

E.g.

// Maybe rename these to `Expect`, `ExpectInline` and `ExpectFile` respectively
pub enum ExpectAny {
    Inline(Expect),
    File(ExpectFile),
}