Allow testing ∃x: p(x): a test passes at least once out of N tries

[Janiczek]

Normal fuzz tests check that a fuzzer passes the test every time (∀x: p(x)).
Sometimes it's helpful to test whether a fuzzer passes the test at least once (∃x: p(x)).

API sketch (with bad naming):

Test.fuzz        : Fuzzer a -> String -> (a -> Expectation) -> Test
Test.fuzzCanPass : Fuzzer a -> String -> (a -> Expectation) -> Test

This can be inefficiently done in userspace like this:

{-| Will run the fuzzer up to 100 times and check if it passes the test at least once.
-}
canPass : Fuzzer a -> (a -> Bool) -> Expectation
canPass fuzzer pred =
    let
        tries =
            100

        go n nLeft =
            if nLeft <= 0 then
                Expect.fail <| "Expected the fuzzer to pass the test at least once out of " ++ String.fromInt tries ++ " tries."

            else
                let
                    seenItPass =
                        Fuzz.examples n fuzzer
                            |> List.any pred
                in
                if seenItPass then
                    Expect.pass

                else
                    go (n + 1) (nLeft - n)
    in
    go 1 tries

since the exposed API lacks the ability to run a single fuzzer with a varying seed. And it's kinda abusing Fuzz.examples.

The library would be able to stop fuzzing as soon as it sees a pass; the userspace version generates in chunks and likely throws some of that work away after it finds a passing value.

[Janiczek]

Alternatively we could expose a function that negates an Expectation, makes failures into passes and vice versa. Then the test "fuzzer X can pass predicate Y at least once" would become "test !Y with fuzzer X doesn't pass"

[Janiczek]

Conceptually Test.fuzz checks ∀x: p(x)
And we want ∃x: p(x)

The original proposal here would give us that directly, while the negation would give us that via the identity ∃x: p(x) === not ∀x: not p(x)

[Janiczek]

Example of where this would be useful: testing Random.Generators: https://github.com/Janiczek/nu-ashworld/blob/2b31f64fb303de4d29d172d704e6d456885954f9/tests/Data/Fight/GeneratorTest.elm#L471-L526