Set readline-relevant ENV vars in interactive_psql(), not caller.

Set readline-relevant ENV vars in interactive_psql(), not caller.

Commit 664d75753 pulled 010_tab_completion.pl's infrastructure for
invoking an interactive psql session out into a generally-useful test
function, but it didn't move enough stuff.  We need to set up various
environment variables that readline will look at, both to ensure
stability of test results and to prevent test actions from cluttering
the calling user's ~/.psql_history.  Expecting calling scripts to
remember to do that is too failure-prone: the other existing caller
001_password.pl did not do it.  Hence, remove those initialization
steps from 010_tab_completion.pl and put them into interactive_psql().
Since interactive_psql was already making a local ENV hash, this has
no effect on calling scripts.

Discussion: https://postgr.es/m/794610.1703182896@sss.pgh.pa.us