Sharded test runs now execute Swift Testing suites, not zero tests

Suite-granularity test sharding now passes -only-testing Target/Suite identifiers to xcodebuild test-without-building instead of rewriting the bundle’s .xctestrun with OnlyTestIdentifiers. Swift Testing suites, which OnlyTestIdentifiers cannot match, are now actually selected per shard, so a successful suite-granularity shard is no longer a false-green run that executed zero tests.