[codex] Add pendingappleupdates table tests

[grahamgilbert]

Summary

• add first tests for pendingappleupdates columns, generator, and plist reader behavior
• make the Software Update plist path configurable for tests
• decode plist data with auto-detection so XML and binary plists are both supported

Validation

• GOCACHE=/tmp/osquery-extension-go-cache go test ./tables/pendingappleupdates
• GOCACHE=/tmp/osquery-extension-go-cache go test ./...