Disable Concurrency/Runtime/async_stream.swift

This test has a race condition which can cause PR testing to fail,
blocking other work.

rdar://78033828 (Swift CI: timeout)

Author: atrick

test/Concurrency/Runtime/async_stream.swift

@@ -11,6 +11,9 @@
 // https://bugs.swift.org/browse/SR-14466
 // UNSUPPORTED: OS=windows-msvc
 
+// Race condition
+// REQUIRES: rdar78033828
+
 import _Concurrency
 import StdlibUnittest
 import Dispatch