Fix test failure with wal_level=minimal.

The newly-added gist pageinspect test prints the LSNs of GiST pages,
expecting them all to be 1 (GistBuildLSN). But with wal_level=minimal,
they got updated by the whole-relation WAL-logging at commit. Fix by
wrapping the problematic tests in the same transaction with the CREATE
INDEX.

Per buildfarm failure on thorntail.

Discussion: https://www.postgresql.org/message-id/3B4F97E5-40FB-4142-8CAA-B301CDFBF982%40iki.fi

contrib/pageinspect/expected/gist.out

+-- The gist_page_opaque_info() function prints the page's LSN. Normally,
+-- that's constant 1 (GistBuildLSN) on every page of a freshly built GiST
+-- index. But with wal_level=minimal, the whole relation is dumped to WAL at
+-- the end of the transaction if it's smaller than wal_skip_threshold, which
+-- updates the LSNs. Wrap the tests on gist_page_opaque_info() in the
+-- same transaction with the CREATE INDEX so that we see the LSNs before
+-- they are possibly overwritten at end of transaction.
+BEGIN;
+-- Create a test table and GiST index.
 CREATE TABLE test_gist AS SELECT point(i,i) p, i::text t FROM
     generate_series(1,1000) i;
 CREATE INDEX test_gist_idx ON test_gist USING gist (p);
@@ -20,6 +29,7 @@ SELECT * FROM gist_page_opaque_info(get_raw_page('test_gist_idx', 2));
  0/1 | 0/0 |         1 | {leaf}
 (1 row)
 
+COMMIT;
 SELECT * FROM gist_page_items(get_raw_page('test_gist_idx', 0), 'test_gist_idx');
  itemoffset |   ctid    | itemlen |       keys        
 ------------+-----------+---------+-------------------

contrib/pageinspect/sql/gist.sql

+-- The gist_page_opaque_info() function prints the page's LSN. Normally,
+-- that's constant 1 (GistBuildLSN) on every page of a freshly built GiST
+-- index. But with wal_level=minimal, the whole relation is dumped to WAL at
+-- the end of the transaction if it's smaller than wal_skip_threshold, which
+-- updates the LSNs. Wrap the tests on gist_page_opaque_info() in the
+-- same transaction with the CREATE INDEX so that we see the LSNs before
+-- they are possibly overwritten at end of transaction.
+BEGIN;
+
+-- Create a test table and GiST index.
 CREATE TABLE test_gist AS SELECT point(i,i) p, i::text t FROM
     generate_series(1,1000) i;
 CREATE INDEX test_gist_idx ON test_gist USING gist (p);
@@ -7,6 +17,8 @@ SELECT * FROM gist_page_opaque_info(get_raw_page('test_gist_idx', 0));
 SELECT * FROM gist_page_opaque_info(get_raw_page('test_gist_idx', 1));
 SELECT * FROM gist_page_opaque_info(get_raw_page('test_gist_idx', 2));
 
+COMMIT;
+
 SELECT * FROM gist_page_items(get_raw_page('test_gist_idx', 0), 'test_gist_idx');
 SELECT * FROM gist_page_items(get_raw_page('test_gist_idx', 1), 'test_gist_idx') LIMIT 5;