Mark test unsupported on Linux and Windows

A few other concurrency tests do so as well.  I will investigate once
those other tests have been resolved.

Author: Julian Lettner

test/Sanitizers/tsan/actor_counters.swift

@@ -4,6 +4,8 @@
 // REQUIRES: concurrency
 // REQUIRES: libdispatch
 // REQUIRES: tsan_runtime
+// UNSUPPORTED: linux
+// UNSUPPORTED: windows
 
 #if canImport(Darwin)
 import Darwin