pgsql: Fix test failure with wal_level=minimal.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5abca4b1cd7193d6a5c8235c97eb240312190bcb

Modified Files
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contrib/pageinspect/expected/gist.out | 10 ++++++++++
contrib/pageinspect/sql/gist.sql      | 12 ++++++++++++
2 files changed, 22 insertions(+)