Suite-granularity sharding selects Swift Testing suites

Suite-granularity test sharding now actually runs Swift Testing suites instead of silently passing with zero tests. Shard selection is delegated to xcodebuild’s -only-testing Target/Suite arguments instead of rewriting the bundle’s .xctestrun with OnlyTestIdentifiers, which only filters XCTest. The downloaded .xctestproducts bundle is no longer mutated, only Tuist-owned temp products are cleaned up.