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/wal.sql

@@ -0,0 +1,20 @@
+--
+-- Validate WAL generation metrics
+--
+
+SET pg_stat_statements.track_utility = FALSE;
+
+CREATE TABLE pgss_wal_tab (a int, b char(20));
+
+INSERT INTO pgss_wal_tab VALUES(generate_series(1, 10), 'aaa');
+UPDATE pgss_wal_tab SET b = 'bbb' WHERE a > 7;
+DELETE FROM pgss_wal_tab WHERE a > 9;
+DROP TABLE pgss_wal_tab;
+
+-- Check WAL is generated for the above statements
+SELECT query, calls, rows,
+wal_bytes > 0 as wal_bytes_generated,
+wal_records > 0 as wal_records_generated,
+wal_records >= rows as wal_records_ge_rows
+FROM pg_stat_statements ORDER BY query COLLATE "C";
+SELECT pg_stat_statements_reset();