test(site-runtime): fix stale SetStaticAttribute tests

HandleSetStaticAttribute was made fire-and-forget (commit 2951507) — it no
longer replies with SetStaticAttributeResponse — but three InstanceActor
tests still ExpectMsg<SetStaticAttributeResponse> and timed out. Verify the
mutation via the GetAttributeRequest round-trip instead, which the FIFO
mailbox makes a sound sync point. Test intent (in-memory update, SQLite
persistence, serialized ordering) is unchanged.

tests/ScadaLink.SiteRuntime.Tests/Actors/InstanceActorIntegrationTests.cs

@@ -123,11 +123,9 @@ public class InstanceActorIntegrationTests : TestKit, IDisposable
                 $"corr-{i}", "Pump1", "Temperature", $"{i}", DateTimeOffset.UtcNow));
         }
 
-        // Wait for all to process
-        for (int i = 0; i < 50; i++)
-        {
-            ExpectMsg<SetStaticAttributeResponse>(TimeSpan.FromSeconds(10));
-        }
+        // SetStaticAttributeCommand is fire-and-forget; the GetAttributeRequest
+        // round-trip below is the sync point — the FIFO mailbox guarantees all
+        // 50 sets are processed before the get is.
 
         // The last value should be the final one
         actor.Tell(new GetAttributeRequest(