Minor: add a test for current_time (no args) (#10509)

Author: alamb

datafusion/sqllogictest/test_files/timestamps.slt

@@ -2795,3 +2795,9 @@ SELECT '2000-12-01 04:04:12' AT TIME ZONE 'America/New York';
 # abbreviated timezone is not supported
 statement error
 SELECT '2023-03-12 02:00:00' AT TIME ZONE 'EDT';
+
+# Test current_time without parentheses
+query B
+select current_time = current_time;
+----
+true