Add support for \aset in pgbench

Add support for \aset in pgbench

This option is similar to \gset, except that it is able to store all
results from combined SQL queries into separate variables.  If a query
returns multiple rows, the last result is stored and if a query returns
no rows, nothing is stored.

While on it, add a TAP test for \gset to check for a failure when a
query returns multiple rows.

Author: Fabien Coelho
Reviewed-by: Ibrar Ahmed, Michael Paquier
Discussion: https://postgr.es/m/alpine.DEB.2.21.1904081914200.2529@lancre