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

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