Unsupported: continuation_validation in backdeploy_concurrency

The 5.5/5.6 concurrency runtime in the concurrency backdeploy library
just doesn't have the pieces necessary to emit the desired error
message. Disabling it in that environment.
Commit afc5116 added the support to track re-used continuations.

Author: etcwilde

test/Concurrency/Runtime/continuation_validation.swift

@@ -7,6 +7,9 @@
 // REQUIRES: concurrency
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
+
+// Commit afc5116ef0de added support for the error check
+// UNSUPPORTED: back_deploy_concurrency
 // UNSUPPORTED: use_os_stdlib
 // UNSUPPORTED: freestanding
 
@@ -29,4 +32,4 @@ import StdlibUnittest
 
     await runAllTestsAsync()
   }
-}
+}