Improve psql tab-completion tests.

Improve psql tab-completion tests.

Fix up recently-added test cases in 010_tab_completion.pl
so that they pass with the rather seriously broken libedit
found in Debian 10 (Buster).

Also, add a few more test cases to improve code coverage.
The total line coverage still looks pretty awful, because
we exercise only a few paths of the giant if-else chain in
psql_completion().  However, this now covers almost all of
the code that isn't in one of those if-blocks.

Discussion: https://postgr.es/m/960764.1643751011@sss.pgh.pa.us