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

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