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

contrib/pg_stat_statements/sql/level_tracking.sql

@@ -97,3 +97,4 @@ SELECT 1 AS "one";
 SELECT 1 + 1 AS "two";
 
 SELECT calls, rows, query FROM pg_stat_statements ORDER BY query COLLATE "C";
+SELECT pg_stat_statements_reset();

contrib/pg_stat_statements/sql/user_activity.sql

@@ -63,3 +63,4 @@ SELECT query, calls, rows FROM pg_stat_statements ORDER BY query COLLATE "C";
 --
 DROP ROLE regress_stats_user1;
 DROP ROLE regress_stats_user2;
+SELECT pg_stat_statements_reset();

contrib/pg_stat_statements/sql/utility.sql

@@ -276,3 +276,4 @@ SET enable_seqscan = on;
 RESET enable_seqscan;
 
 SELECT calls, rows, query FROM pg_stat_statements ORDER BY query COLLATE "C";
+SELECT pg_stat_statements_reset();