Refactor more the regression tests of pg_stat_statements

This commit expands more the refactoring of the regression tests of
pg_stat_statements, with tests moved out of pg_stat_statements.sql into
separate files.  The following file structure is now used:
- select is mostly the former pg_stat_statements.sql, renamed.
- dml for INSERT/UPDATE/DELETE and MERGE
- user_activity, to test role-level checks and stat resets.
- wal, to check the WAL generation after some queries.

Like e8dbdb1, there is no change in terms of code coverage or results,
and this finishes the split I was aiming for in these tests.  Most of
the tests used "test" of "pgss_test" as names for the tables used, these
are renamed to less generic names.

Reviewed-by: Bertrand Drouvot
Discussion: https://postgr.es/m/Y/7Y9U/y/keAW3qH@paquier.xyz

contrib/pg_stat_statements/sql/user_activity.sql

@@ -0,0 +1,65 @@
+--
+-- Track user activity and reset them
+--
+
+SET pg_stat_statements.track_utility = TRUE;
+SELECT pg_stat_statements_reset();
+CREATE ROLE regress_stats_user1;
+CREATE ROLE regress_stats_user2;
+
+SET ROLE regress_stats_user1;
+
+SELECT 1 AS "ONE";
+SELECT 1+1 AS "TWO";
+
+RESET ROLE;
+SET ROLE regress_stats_user2;
+
+SELECT 1 AS "ONE";
+SELECT 1+1 AS "TWO";
+
+RESET ROLE;
+SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+
+--
+-- Don't reset anything if any of the parameter is NULL
+--
+SELECT pg_stat_statements_reset(NULL);
+SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+
+--
+-- remove query ('SELECT $1+$2 AS "TWO"') executed by regress_stats_user2
+-- in the current_database
+--
+SELECT pg_stat_statements_reset(
+	(SELECT r.oid FROM pg_roles AS r WHERE r.rolname = 'regress_stats_user2'),
+	(SELECT d.oid FROM pg_database As d where datname = current_database()),
+	(SELECT s.queryid FROM pg_stat_statements AS s
+				WHERE s.query = 'SELECT $1+$2 AS "TWO"' LIMIT 1));
+SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+
+--
+-- remove query ('SELECT $1 AS "ONE"') executed by two users
+--
+SELECT pg_stat_statements_reset(0,0,s.queryid)
+	FROM pg_stat_statements AS s WHERE s.query = 'SELECT $1 AS "ONE"';
+SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+
+--
+-- remove query of a user (regress_stats_user1)
+--
+SELECT pg_stat_statements_reset(r.oid)
+		FROM pg_roles AS r WHERE r.rolname = 'regress_stats_user1';
+SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+
+--
+-- reset all
+--
+SELECT pg_stat_statements_reset(0,0,0);
+SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
+
+--
+-- cleanup
+--
+DROP ROLE regress_stats_user1;
+DROP ROLE regress_stats_user2;