Actually run Swift Testing suites in suite-granularity sharding

Suite-granularity test sharding no longer rewrites the test products bundle’s .xctestrun with OnlyTestIdentifiers. It now derives -only-testing Target/Suite identifiers and passes them through xcodebuild test-without-building, which selects both XCTest and Swift Testing tests at the suite level. Previously, every sharded run on Swift Testing test suites completed instantly while running zero tests.