Update SyntheticSourceIndexSettingsProviderTests to expect source mode deprecation warning

Author: ChrisHegarty

x-pack/plugin/logsdb/src/test/java/org/elasticsearch/xpack/logsdb/SyntheticSourceIndexSettingsProviderTests.java

@@ -24,6 +24,7 @@
 
 import java.io.IOException;
 import java.time.Instant;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -402,4 +403,11 @@ public void testGetAdditionalIndexSettingsDowngradeFromSyntheticSourceFileMatch(
         assertThat(result.size(), equalTo(0));
         assertThat(newMapperServiceCounter.get(), equalTo(0));
     }
+
+    protected List<String> filteredWarnings() {
+        var warnings = super.filteredWarnings();
+        List<String> result = warnings == null ? new ArrayList<>() : new ArrayList<>(warnings);
+        result.add("Configuring source mode in mappings is deprecated and will be removed in future versions");
+        return result;
+    }
 }