Suite shard destination resolved from selected test plan

Suite-granularity test sharding now resolves the right concrete simulator destination when the user selects a specific test plan. The chosen test plan is threaded through destination inference and concretization, so multi-platform schemes with plans like MacOnly + iOS get platform=iOS Simulator,id=<udid> for sharded enumeration instead of platform=iOS Simulator, avoiding Xcode’s exit-code-70 rejection.