psql: Add tests for repeated calls of \bind[_named]

The implementation assumes that on multiple calls of these meta-commands
the last one wins.  Multiple \g calls in-between mean multiple
executions.

There were no tests to check these properties, hence let's add
something.

Author: Jelte Fennema-Nio, Michael Paquier
Discussion: https://postgr.es/m/CAGECzQSTE7CoM=Gst56Xj8pOvjaPr09+7jjtWqTC40pGETyAuA@mail.gmail.com

src/test/regress/sql/psql.sql

@@ -63,6 +63,10 @@ SELECT $1, $2 \parse stmt3
 \bind_named stmt4
 \bind_named
 \g
+-- Last \bind_named wins
+\bind_named stmt2 'foo' \bind_named stmt3 'foo2' 'bar2' \g
+-- Multiple \g calls mean multiple executions
+\bind_named stmt2 'foo3' \g \bind_named stmt3 'foo4' 'bar4' \g
 
 -- \close (extended query protocol)
 \close
@@ -76,6 +80,11 @@ SELECT 1 \bind \g
 SELECT $1 \bind 'foo' \g
 SELECT $1, $2 \bind 'foo' 'bar' \g
 
+-- last \bind wins
+select $1::int as col \bind 'foo' \bind 2 \g
+-- Multiple \g calls mean multiple executions
+select $1::int as col \bind 1 \g \bind 2 \g
+
 -- errors
 -- parse error
 SELECT foo \bind \g