Suite sharding uses the concrete simulator matching the selected test plan

Suite-level test sharding now threads the requested test plan through its destination resolution, so tuist test -d builds enumerate tests against the same concrete iOS simulator destination as the build-for-testing step. Schemes with multiple test plans (such as a macOS-only and an iOS plan) correctly map the requested plan to a concrete simulator instead of falling back to a platform-only destination that Xcode rejects.