Don't try to force TERM to a fixed value in tab-completion test.
Right at the moment, this is making things worse not better in the
buildfarm. I'm not happy with anything about the current state,
but let's at least try to have a green buildfarm report while further
investigation continues.
Discussion: https://postgr.es/m/23181.
1578167938@sss.pgh.pa.us