Test: AcquiredHeaderVersionTest fails when the 3.15.0 extraction has no ios-arm64 patchlevel #313

Seen while verifying #307, unrelated to it. Fails in a worktree whose python-multiplatform/build/python-standalone/extracted/3.15.0 holds android/linux/macos/windows and an ios dir (Python.xcframework, testbed) where AcquiredHeaderVersion.patchlevelFile finds no ios-arm64 header: 'iOS headers missing under ../python-multiplatform/build/python-standalone/extracted/3.15.0'. It passed earlier in the same worktree, before other targets were extracted (the test skips when nothing is present), so it depends on build-directory state. Completion: the test either finds the 3.15.0 iOS layout or explains which download is missing; deterministic.