Gate Clock Runtime Test on Current Runtimes

The back deployment runtime does not contain the symbols necessary
to execute this test. Mark it unsupported.

Author: CodaFi

test/Concurrency/Runtime/clock.swift

@@ -4,6 +4,10 @@
 // REQUIRES: executable_test
 // REQUIRES: concurrency_runtime
 
+// Test requires _swift_task_enterThreadLocalContext which is not available 
+// in the back deployment runtime.
+// UNSUPPORTED: back_deployment_runtime
+
 import _Concurrency
 import StdlibUnittest
 
@@ -104,4 +108,4 @@ var tests = TestSuite("Time")
 
     await runAllTestsAsync()
   }
-}
+}