Suite sharding selects Swift Testing suites

Suite-granularity test sharding now passes the resolved per-shard suites through xcodebuild’s -only-testing arguments (Target/Suite) instead of rewriting the downloaded test products bundle’s .xctestrun. OnlyTestIdentifiers in an .xctestrun only filters XCTest, so the previous path silently completed runs with zero executions on pure Swift Testing suites. The new approach works for both XCTest and Swift Testing and leaves the downloaded products untouched, removing the false-green successful runs that ran nothing.