Test plan-aware simulator destination for suite sharding

The shard destination resolver now threads the requested test plan through the entire planning path. For schemes that contain multiple test plans on different platforms, suite-level test enumeration uses the same test target and simulator destination as the actual build-for-testing invocation, so an iOS-only plan with a sibling macOS-only plan reliably resolves to a concrete iOS simulator destination instead of falling back to a generic platform, which Xcode would reject with exit code 70.