pgsql: Improve cleanup phases in regression tests of pg_stat_statements

Improve cleanup phases in regression tests of pg_stat_statements

As shaped, two DROP ROLE queries included in "user_activity" were
showing in the reports for "wal".  The intention is to keep each test
isolated and independent, so this is incorrect.  This commit adds some
calls to pg_stat_statements_reset() to clean up the statistics once each
test finishes, so as there are no risks of overlap in the reports for
individial scenarios.

The addition in "user_activity" fixes the output of "wal".  The new
resets done in "level_tracking" and "utility" are added for consistency
with the rest, though they do not affect the stats generated in the
other tests.

Oversight in d0028e3.

Reported-by: Andrei Zubkov
Discussion: https://postgr.es/m/7beb722dd016bf54f1c78bfd6d44a684e28da624.camel@moonset.ru

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9a714b9d6ec0a562bc9242ec17122321cd174588

Modified Files
--------------
.../pg_stat_statements/expected/level_tracking.out    |  6 ++++++
contrib/pg_stat_statements/expected/user_activity.out |  6 ++++++
contrib/pg_stat_statements/expected/utility.out       |  6 ++++++
contrib/pg_stat_statements/expected/wal.out           | 19 ++++++++-----------
contrib/pg_stat_statements/sql/level_tracking.sql     |  1 +
contrib/pg_stat_statements/sql/user_activity.sql      |  1 +
contrib/pg_stat_statements/sql/utility.sql            |  1 +
7 files changed, 29 insertions(+), 11 deletions(-)