Remove async from the main function for the time being.

This is causing some of our tests to terminate abnormally with an error out of the Swift runtime: "freed pointer was not the last allocation". We're going to need `async` eventually to support swift-testing, but in the meantime, this fixes the broken projects while I investigate further.

PiperOrigin-RevId: 662923954

tools/test_discoverer/TestDiscoverer.swift

@@ -109,7 +109,7 @@ struct TestDiscoverer: ParsableCommand {
       \(availabilityAttribute)
       @main
       struct Main {
-        static func main() async {
+        static func main() {
           do {
             try XCTestRunner.run()