pgsql: pg_stat_statements: Add tests for nested queries with level trac

pg_stat_statements: Add tests for nested queries with level tracking

There have never been any regression tests in PGSS for various query
patterns for nested queries combined with level tracking, like:
- Multi-statements.
- CREATE TABLE AS
- CREATE/REFRESH MATERIALIZED VIEW
- DECLARE CURSOR
- EXPLAIN, with a subset of the above supported.
- COPY.

All the tests added here track historical, sometimes confusing, existing
behaviors. For example, EXPLAIN stores two PGSS entries with the same
top-level query string but two different query IDs as one is calculated
for the top-level EXPLAIN (this part is right) and a second one for the
inner query in the EXPLAIN (this part is not right).

A couple of patches are under discussion to improve the situation, and
all the tests added here will prove useful to evaluate the changes
discussed.

Author: Anthonin Bonnefoy
Reviewed-by: Michael Paquier, Jian He
Discussion: https://postgr.es/m/CAO6_XqqM6S9bQ2qd=75W+yKATwoazxSNhv5sjW06fjGAtHbTUA@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/45e0ba30fc40581f320fac17ad8b4e0676e1b3b5

Modified Files
--------------
.../pg_stat_statements/expected/level_tracking.out | 1151 ++++++++++++++++++++
contrib/pg_stat_statements/sql/level_tracking.sql | 246 +++++
2 files changed, 1397 insertions(+)