Add more stddev/var tests cases

Add tests cases for stddev_samp/pop and var_smap/pop the includes a
group_by clause.

datafusion/sqllogictest/test_files/aggregate.slt

@@ -500,6 +500,85 @@ select stddev(sq.column1) from (values (1.1), (2.0), (3.0)) as sq
 ----
 0.950438495292
 
+# csv_query_stddev_7
+query IR
+SELECT c2, stddev_samp(c12) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+----
+1 0.303641032262
+2 0.284581967411
+3 0.296002660506
+4 0.284324609109
+5 0.331034486752
+
+# csv_query_stddev_8
+query IR
+SELECT c2, stddev_pop(c12) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+----
+1 0.296659845456
+2 0.278038978602
+3 0.288107833475
+4 0.278074953424
+5 0.318992813225
+
+# csv_query_stddev_9
+query IR
+SELECT c2, var_pop(c12) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+----
+1 0.088007063906
+2 0.077305673622
+3 0.083006123709
+4 0.077325679722
+5 0.101756414889
+
+# csv_query_stddev_10
+query IR
+SELECT c2, var_samp(c12) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+----
+1 0.092197876473
+2 0.080986896176
+3 0.087617575027
+4 0.080840483345
+5 0.109583831419
+
+# csv_query_stddev_11
+query IR
+SELECT c2, var_samp(c12) FROM aggregate_test_100 WHERE c12 > 0.90 GROUP BY c2 ORDER BY c2
+----
+1 0.000889240174
+2 0.000785878272
+3 NULL
+4 NULL
+5 0.000269544643
+
+# Use PostgresSQL dialect
+statement ok
+set datafusion.sql_parser.dialect = 'Postgres';
+
+# csv_query_stddev_12
+query IR
+SELECT c2, var_samp(c12) FILTER (WHERE c12 > 0.90) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+----
+1 0.000889240174
+2 0.000785878272
+3 NULL
+4 NULL
+5 0.000269544643
+
+# Restore the default dialect
+statement ok
+set datafusion.sql_parser.dialect = 'Generic';
+
+# csv_query_stddev_13
+query IR
+SELECT c2, var_samp(CASE WHEN c12 > 0.90 THEN c12 ELSE null END) FROM aggregate_test_100 GROUP BY c2 ORDER BY c2
+----
+1 0.000889240174
+2 0.000785878272
+3 NULL
+4 NULL
+5 0.000269544643
+
+
 # csv_query_approx_median_1
 query I
 SELECT approx_median(c2) FROM aggregate_test_100