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@gmail.com>
Discussion: https://postgr.es/m/CAA5RZ0uK1PSrgf52bWCtDpzaqbWt04o6ZA7zBm6UQyv7vyvf9w@mail.gmail.com