ESQL: Enhance test assertion in testSimpleFinishClose (#100971)

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/operator/OperatorTestCase.java

@@ -224,7 +224,7 @@ public abstract class OperatorTestCase extends AnyOperatorTestCase {
     public void testSimpleFinishClose() {
         DriverContext driverContext = driverContext();
         List<Page> input = CannedSourceOperator.collectPages(simpleInput(driverContext.blockFactory(), 1));
-        assert input.size() == 1 : input.size();
+        assert input.size() == 1 : "Expected single page, got: " + input;
         // eventually, when driverContext always returns a tracking factory, we can enable this assertion
         // assertThat(driverContext.blockFactory().breaker().getUsed(), greaterThan(0L));
         Page page = input.get(0);