Disable clock test on backdeploy concurrency lib

Author: etcwilde

test/Concurrency/Runtime/clock.swift

@@ -7,6 +7,7 @@
 // Test requires _swift_task_enterThreadLocalContext which is not available 
 // in the back deployment runtime.
 // UNSUPPORTED: back_deployment_runtime
+// UNSUPPORTED: back_deploy_concurrency
 
 import _Concurrency
 import StdlibUnittest