small regression adjustment

It occurred to me after having to change I think 9 files to clean up a
small mess in the jsonb regression tests the other day that we might
usefully expose the inputdir and outputdir to psql as variables when
running pg_regress. Then we might be able to do thing like this, quite
independent of location:

\set datafile :inputdir/data/mystuff.data
COPY mytable FROM :'datafile';

The actual location would not need to appear in the SQL (and thus not in
the results file either).

A small patch that tries to do that is attached.

Thoughts?

cheers

andrew