Resolve suite shard destination from the selected test plan

When a scheme has multiple test plans, the suite sharding destination resolver can now look at the test plan you actually requested. The resolver carries that plan through while inferring the platform and concretizing the destination, so an iOS-only test plan resolves to a concrete simulator destination such as platform=iOS Simulator,id=... instead of falling back to a generic platform-only destination that Xcode used to reject during test enumeration.