Add a regression test for pgstattuple.

This is mainly to prove that the NaN fix actually works cross-platform.

contrib/pgstattuple/sql/pgstattuple.sql

@@ -0,0 +1,17 @@
+CREATE EXTENSION pgstattuple;
+
+--
+-- It's difficult to come up with platform-independent test cases for
+-- the pgstattuple functions, but the results for empty tables and
+-- indexes should be that.
+--
+
+create table test (a int primary key);
+
+select * from pgstattuple('test'::text);
+select * from pgstattuple('test'::regclass);
+
+select * from pgstatindex('test_pkey');
+
+select pg_relpages('test');
+select pg_relpages('test_pkey');