[test] Disable isolated_deinit_main_sync.swift for

back-deployment

This test relies on `_swift_task_deinitOnExecutor`, which
is only available in newer runtimes.

Author: hamishknight

test/Concurrency/Runtime/isolated_deinit_main_sync.swift

@@ -3,6 +3,7 @@
 // REQUIRES: executable_test
 // REQUIRES: concurrency
 // REQUIRES: concurrency_runtime
+// UNSUPPORTED: back_deployment_runtime
 
 var isDead: Bool = false