Add Decimal256 sqllogictests for SUM, MEDIAN and COUNT aggregate expr…

…essions (#7889)

* Add Decimal256 sqllogictests test for SUM aggregate

* Add median and avg sqllogictests tests

* Remove duplicate

* Add count test

datafusion/sqllogictest/test_files/decimal.slt

@@ -701,5 +701,24 @@ select arrow_typeof(max(c1)), max(c1) from decimal256_simple where c4=false;
 ----
 Decimal256(50, 6) 0.00005
 
+query TR
+select arrow_typeof(sum(c1)), sum(c1) from decimal256_simple;
+----
+Decimal256(60, 6) 0.00055
+
+query TR
+select arrow_typeof(median(c1)), median(c1) from decimal256_simple;
+----
+Decimal256(50, 6) 0.00004
+
+query IR
+select count(*),c1 from decimal256_simple group by c1 order by c1;
+----
+1 0.00001
+2 0.00002
+3 0.00003
+4 0.00004
+5 0.00005
+
 statement ok
 drop table decimal256_simple;