Disabling failing test Concurrency/Runtime/clock.swift on cooperative executor

This is XFAILed on cooperative executor due to missing implementations

Author: kateinoigakukun

test/Concurrency/Runtime/clock.swift

@@ -9,6 +9,9 @@
 // UNSUPPORTED: back_deployment_runtime
 // UNSUPPORTED: back_deploy_concurrency
 
+// This is XFAILed on cooperative executor due to missing implementations
+// XFAIL: single_threaded_runtime
+
 import _Concurrency
 import StdlibUnittest