Add lazy version of Expect.onFail

[jfmengels]

Expect.onFail allows users to replace the failure message for a given test failure. From the documentation:

"something"
    |> Expect.equal "something else"
    |> Expect.onFail "thought those two strings would be the same"

The problem with this is that failure message is computed eagerly and not lazily.
In the elm-review testing library, some of the assertions compute a fairly complex failure message which can be slow to generate. Being able to do it lazily can make the tests run much faster.

In practice, this is already possible through a custom function like this:

onFailLazy : (() -> String) -> Expectation -> Expectation
onFailLazy message expectation =
    case Test.Runner.getFailureReason expectation of
        Just _ ->
            expectation |> Expect.onFail (message ())

        Nothing ->
            expectation

but this is very tricky to figure out by yourself, as Test.Runner.getFailureReason is necessary and it isn't showcased much (for good reasons).

I would like to propose that this function makes it in the library (maybe under a different name, I'm not attached to onFailLazy at all).

---

When mentioning this on Slack, @Janiczek mentioned that there are also places where the initial error message would be good to keep. In which case, we could make a similar function to the above available where the failure reason received from Test.Runner.getFailureReason is not ignored (or just have one function for both) (cc @edkelly303 who first mentioned it AFAIK)

onFailure : (String -> String) -> Expectation -> Expectation

[edkelly303]

When mentioning this on Slack, @Janiczek mentioned that there are also places where the initial error message would be good to keep. In which case, we could make a similar function to the above available where the failure reason received from Test.Runner.getFailureReason is not ignored (or just have one function for both) (cc @edkelly303 who first mentioned it AFAIK)

Yes, thanks Jeroen - this is tracked in issue #234