tests: disable Concurrency/Runtime/objc_async.swift because it crashes sometimes.

rdar://75783864

Author: eeckstein

test/Concurrency/Runtime/objc_async.swift

@@ -7,6 +7,9 @@
 // REQUIRES: concurrency
 // REQUIRES: objc_interop
 
+// This test is flaky and crashes sometimes, mostly when testing with swift_test_mode_optimize.
+// REQUIRES: rdar75783864
+
 
 @main struct Main {
   static func main() async {