Add regression test coverage for contrib/pg_buffercache.

We can't check the output of this view very closely without
creating portability headaches, but we can make sure that
the number of rows is as-expected.  In any case, this is
sufficient to exercise all the C code within, which is a
lot better than the 0% coverage we had before.

DongWook Lee

Discussion: https://postgr.es/m/CAAcByaLCHGJB7qAENEcx9D09UL=w4ma+yijwF_-1MSqQZ9wK6Q@mail.gmail.com

contrib/pg_buffercache/.gitignore

@@ -0,0 +1,4 @@
+# Generated subdirectories
+/log/
+/results/
+/tmp_check/

contrib/pg_buffercache/Makefile

@@ -10,6 +10,8 @@ DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \
 	pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql
 PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time"
 
+REGRESS = pg_buffercache
+
 ifdef USE_PGXS
 PG_CONFIG = pg_config
 PGXS := $(shell $(PG_CONFIG) --pgxs)

contrib/pg_buffercache/expected/pg_buffercache.out

@@ -0,0 +1,10 @@
+CREATE EXTENSION pg_buffercache;
+select count(*) = (select setting::bigint
+                   from pg_settings
+                   where name = 'shared_buffers')
+from pg_buffercache;
+ ?column? 
+----------
+ t
+(1 row)
+

contrib/pg_buffercache/sql/pg_buffercache.sql

@@ -0,0 +1,6 @@
+CREATE EXTENSION pg_buffercache;
+
+select count(*) = (select setting::bigint
+                   from pg_settings
+                   where name = 'shared_buffers')
+from pg_buffercache;