[test] Restrict test/Concurrency/voucher_propagation.swift

This is failing (possibly non-deterministically) on some device
platforms, so restrict it to macOS until we fix it.

rdar://83459813

Author: benlangmuir

test/Concurrency/voucher_propagation.swift

@@ -12,6 +12,9 @@
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
 
+// rdar://83459813 - assertion failure in TaskGroup.cpp
+// REQUIRES: OS=macosx
+
 import Darwin
 import Dispatch
 import StdlibUnittest