replace deprecated event.set with event.setIfInitialized

Signed-off-by: Emmanuel Ferdman <emmanuelferdman@gmail.com>

Author: emmanuel-ferdman

druntime/test/shared/src/dllgc.d

@@ -29,7 +29,7 @@ version(DLL)
         void term() nothrow
         {
             stop = true;
-            event.set();
+            event.setIfInitialized();
             joinLowLevelThread(tid);
         }
     }