Fix race condition in stats.sql added in 5264add784

Very occasionally the stats test failed due to the number of sessions not
being updated yet. Likely this requires that there is contention on the
database's stats entry. Solve this by forcing pending stats to be flushed
before fetching the stats.

I verified that there are no other test failures after making
pgstat_report_stat() only flush stats when force = true.

Per message from Tom Lane and buildfarm member crake.

Discussion: https://postgr.es/m/3428246.1663271992@sss.pgh.pa.us
Backpatch: 15-, where 5264add784 added the test

src/test/regress/expected/stats.out

@@ -560,6 +560,12 @@ DROP TABLE prevstats;
 -- Test that sessions is incremented when a new session is started in pg_stat_database
 SELECT sessions AS db_stat_sessions FROM pg_stat_database WHERE datname = (SELECT current_database()) \gset
 \c
+SELECT pg_stat_force_next_flush();
+ pg_stat_force_next_flush 
+--------------------------
+ 
+(1 row)
+
 SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELECT current_database());
  ?column? 
 ----------

src/test/regress/sql/stats.sql

@@ -294,6 +294,7 @@ DROP TABLE prevstats;
 -- Test that sessions is incremented when a new session is started in pg_stat_database
 SELECT sessions AS db_stat_sessions FROM pg_stat_database WHERE datname = (SELECT current_database()) \gset
 \c
+SELECT pg_stat_force_next_flush();
 SELECT sessions > :db_stat_sessions FROM pg_stat_database WHERE datname = (SELECT current_database());
 
 -- Test pg_stat_bgwriter checkpointer-related stats, together with pg_stat_wal