tests: yield in the shared-string concurrency barrier

Author: Moonbot-Tech

RTL-test/semantic/string_refcount_concurrency_semantic.dpr

@@ -37,7 +37,7 @@ end;
 procedure SpinUntil(var Value: LongInt; Expected: LongInt); inline;
 begin
   while AtomicRead(Value) < Expected do
-    ;
+    TThread.Yield;
 end;
 
 procedure TCopyThread.Execute;