New canGenerateNewSnapshots to avoid automatically creating missing ones in CI

[NachoSoto]

Fixes #748.

[NachoSoto]

Bump?

[CraigSiemens]

I had a related idea and stumbled across this searching issues for it.

I was looking for a way to record only the snapshots for the failing tests to avoid re-recording all the snapshots in a project. Mainly to avoid unrelated changed being applied and the back and forth of intel/apple silicon shaping out images.

---

This is just an idea, feel free to take or leave it. We could introduce a recordingMode rather than having more bools in the tests/globally.

enum RecordingMode {
    /// Only record a snapshot when there isn't an existing one. The default behaviour.
    case missingSnapshots
    
    /// Record the snapshots only for failing tests. Useful for when a change affects many
    /// snapshots across a project but you don't want the snapshots for the passing tests
    /// to be recorded as well.
    case failingTests
    
    /// Always record snapshots. The same as setting `isRecording` to true.
    case always
    
    /// Never record a snapshot. The same as setting `canGenerateNewSnapshots` to true.
    case never
}

This could be introduced without being a breaking change by modifying isRecording to be

public var isRecording: Bool {
   get { recordingMode == .always }
   set { recordingMode = newValue ? .always : .missingSnapshot }
}