pgsql: Add convenience view to stats import test.

Add convenience view to stats import test.

Presently, many statements in stats_import.sql select all columns
from the pg_stats system view. A proposed follow-up commit would
add columns to this view (some of which are not stable across test
runs), breaking all of these tests. This commit introduces a
convenience view for those statements so that future changes are
minimally disruptive.

Author: Corey Huinker <corey(dot)huinker(at)gmail(dot)com>
Reviewed-by: Sami Imseih <samimseih(at)gmail(dot)com>
Discussion: https://postgr.es/m/CADkLM%3DcoCVy92QkVUUTLdo5eO2bMDtwMrzRn_8miAhX%2BuPaqXg%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/233bbdf031e4832a409e3028b45d709e7ddb93c9

Modified Files
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src/test/regress/expected/stats_import.out | 65 ++++++++++++++++++------------
src/test/regress/sql/stats_import.sql | 61 +++++++++++++++++-----------
2 files changed, 76 insertions(+), 50 deletions(-)