Fix ignored Future in TestDeltaLakePageSink

It's to be future-proof and also suppress an error-prone warning.
Currently, the returned future is always already completed.

plugin/trino-delta-lake/src/test/java/io/trino/plugin/deltalake/TestDeltaLakePageSink.java

@@ -43,6 +43,7 @@
 import java.util.Collection;
 import java.util.List;
 import java.util.Optional;
+import java.util.concurrent.TimeUnit;
 
 import static com.google.common.collect.ImmutableList.toImmutableList;
 import static com.google.common.io.MoreFiles.deleteRecursively;
@@ -99,7 +100,7 @@ public void testPageSinkStats()
                 }
             }
             Page page = pageBuilder.build();
-            pageSink.appendPage(page);
+            pageSink.appendPage(page).get(10, TimeUnit.SECONDS);
 
             JsonCodec<DataFileInfo> dataFileInfoCodec = new JsonCodecFactory().jsonCodec(DataFileInfo.class);
             Collection<Slice> fragments = getFutureValue(pageSink.finish());