Suite shard destination resolves from the selected test plan

Suite-granularity test sharding now correctly handles schemes with multiple test plans pointing at different platforms. The selected test plan is threaded through suite shard destination resolution, so platform=iOS Simulator destinations become concrete (platform=iOS Simulator,id=…) when an iOS-only plan is requested. Previously, the resolver could pick the wrong test target and fall back to a generic destination that Xcode rejected with exit code 70.