Add regression tests for the iOS SwiftPM tooling

Adds a shared invariant checker for the files the SwiftPM tooling
generates (machine paths, scheme script, Package.swift names and
dependencies, deployment target, pbxproj integrity), and a lifecycle
test that runs scaffold, add, sync, update and deinit on a fake app
on disk and checks the invariants and byte/inode identity after each
step. Test-only; runs offline in the Linux Jest job.

Author: chrfalch