pg_stat_statements: add missing tests for nesting_level

Hi,

While looking at pg_stat_statements nesting_level, I realized that there
are missing nesting_level tests for pgss_planner and pgss_ExecutorFinish.
That is, if you remove nesting_level++ and nesting_level-- in those 2 hooks,
the tests will still succeed.

For pgss_planner the nesting_level updates missing tests are the ones
when track_planning is enabled.

Attached is a quick patch to add coverage.


--
Sami Imseih
Amazon Web Services (AWS)