Suite sharding now actually runs Swift Testing suites

Suite-granularity test sharding was rewriting each shard’s selection into the products bundle’s .xctestrun as OnlyTestIdentifiers, a key that only filters XCTest. Acceptance suites are pure Swift Testing, so a shard with a successfully downloaded products bundle was launching the test host, matching zero Swift Testing tests, and exiting 0: a false green. Sharding now passes xcodebuild’s -only-testing arguments (Target/Suite for suite granularity, Target for module granularity) instead of touching the .xctestrun, so Swift Testing suites actually run in shards alongside XCTest classes.