snapshot-testing

While there are several overloads of the Add() method having Func<Task> callback parameter, the ones accepting Func<object, Task> callback or Func<TValue, Task> callback are just missing - the non-async versions using Action<object> callback and Action<TValue> callback exist, though.

Specifically the Swift version of the snapshot method in the iOSSnapshotTestCase subspec. The Objective-C version of this behaves correctly.

To reproduce:

func testView() {
    let view = UIView(frame: .init(x: 0, y: 0, width: 100, height: 100))
    SnapshotVerifyWithInvertedColors(view, identifier: "someIdentifier")
}

When run in record mode, this should produce a referen

The built in error message is very opaque (necessary because it has to be generic) but users can likely provide a more helpful friendly message customised to their tests (or e.g. when the diff just isn't useful). Therefore an option to customise this message would be useful.

This would only really be usable once #28 is implemented.