pgsql: pg_stat_statements: Add more tests for level tracking

pg_stat_statements: Add more tests for level tracking

This commit adds tests to verify the computation of the nesting level
for two code paths: the planner hook and the ExecutorFinish() hook. The
nesting level is essential to save a correct "toplevel" status for the
added PGSS entries.

The author has noticed that removing the manipulations of nesting_level
in these two code paths did not cause the tests to complain, meaning
that we never had coverage for the assumptions taken by the code.

Author: Sami Imseih <samimseih(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAA5RZ0uK1PSrgf52bWCtDpzaqbWt04o6ZA7zBm6UQyv7vyvf9w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1572ea96e65731eee3227fbc3c119bd9f39e8e50

Modified Files
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.../pg_stat_statements/expected/level_tracking.out | 66 ++++++++++++++++++++++
contrib/pg_stat_statements/sql/level_tracking.sql | 38 +++++++++++++
2 files changed, 104 insertions(+)