Add support for installing StagehandTesting via Swift Package Manager

[NickEntin]

[NickEntin]

Supporting SPM for StagehandTesting is blocked by uber/ios-snapshot-test-case#91, since StagehandTesting has a dependency on iOSSnapshotTestCase. We could potentially add support for only Stagehand now, and follow up with StagehandTesting once that dependency is available.

[NickEntin]

It looks like SPM will support publishing multiple libraries in the same package. One extra piece of complexity here is that StagehandTesting needs to be able to @testable import Stagehand. That is handled in Cocoapods here, but I'm not sure how it will work with SPM.

[NickEntin]

This has been partially resolved by #15, and Stagehand can now be installed via SPM. This ticket is still open to track the remaining work to make StagehandTesting work as well.

[NickEntin]

This will be a bit easier after landing #54, since SnapshotTesting already supports SPM.

[NickEntin]

I have a branch that I think should resolve this. I'm trying to test it, but my sample SPM project keeps crashing Xcode. 😞