Suppress less info in regression tests using DROP CASCADE.

DROP CASCADE doesn't currently promise to visit dependent objects in
a fixed order, so when the regression tests use it, we typically need
to suppress the details of which objects get dropped in order to have
predictable test output.  Traditionally we've done that by setting
client_min_messages higher than NOTICE, but there's a better way:
we can "\set VERBOSITY terse" in psql.  That suppresses the DETAIL
message with the object list, but we still get the basic notice telling
how many objects were dropped.  So at least the test case can verify
that the expected number of objects were dropped.

The VERBOSITY method was already in use in a few places, but run
around and use it wherever it makes sense.

Discussion: https://postgr.es/m/10766.1501608885@sss.pgh.pa.us

src/test/regress/sql/create_index.sql

@@ -1083,6 +1083,5 @@ REINDEX SCHEMA schema_to_reindex;
 -- Clean up
 RESET ROLE;
 DROP ROLE regress_reindexuser;
-SET client_min_messages TO 'warning';
+\set VERBOSITY terse \\ -- suppress cascade details
 DROP SCHEMA schema_to_reindex CASCADE;
-RESET client_min_messages;