add struct testcase

Author: jiangzhx

datafusion/core/tests/sqllogictests/test_files/functions.slt

@@ -418,3 +418,23 @@ SELECT length(c1) FROM test
 
 statement ok
 drop table test
+
+statement ok
+CREATE EXTERNAL TABLE simple_struct_test (
+  a INT,
+  b INT,
+  c INT
+)
+STORED AS CSV
+WITH HEADER ROW
+LOCATION './tests/data/example.csv'
+
+# generic query length
+query ?
+SELECT struct(a,b,c) from simple_struct_test
+----
+{c0: 1, c1: 2, c2: 3}
+
+statement ok
+drop table simple_struct_test
+