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.huinker@gmail.com>
Reviewed-by: Sami Imseih <samimseih@gmail.com>
Discussion: https://postgr.es/m/CADkLM%3DcoCVy92QkVUUTLdo5eO2bMDtwMrzRn_8miAhX%2BuPaqXg%40mail.gmail.com