Add validation test for CurrentTimestamp

Author: Mariam-Almesfer

backends-velox/src/test/scala/org/apache/gluten/functions/ScalarFunctionsValidateSuite.scala

@@ -1521,4 +1521,14 @@ abstract class ScalarFunctionsValidateSuite extends FunctionsValidateSuite {
         }
     }
   }
+
+  test("current_timestamp") {
+    withSQLConf("spark.sql.optimizer.excludedRules" -> "") {
+      runQueryAndCompare("SELECT current_timestamp()") {
+        df =>
+          checkGlutenPlan[ProjectExecTransformer](df)
+          checkFallbackOperators(df, 0)
+      }
+    }
+  }
 }