Suite-granularity test sharding runs Swift Testing suites, not just XCTest

Suite-granularity test sharding now selects Swift Testing suites. Shard selection was previously written into each shard’s xctestrun as OnlyTestIdentifiers, which only filters XCTest tests; shards run against Swift Testing bundles matched zero tests and exited 0, reporting success while running nothing. Selection is now derived from the shard’s modules/suites and passed to xcodebuild as -only-testing arguments (Target/Suite for suite granularity, Target for module granularity), which filter both XCTest and Swift Testing tests. Downloaded/extracted test product bundles are no longer rewritten.