Reduce maintenance burden of alternative output files with \if \quit

Hi,

We have a few alterntive expected output files that are essentially full
of errors, because a certain feature isn't supported. Those are somewhat
painful to maintain. I wonder if it'd be a good idea to reduce the
maintenance overhead for some of them by putting something like

SELECT NOT feature_check_expr() AS dont_have_feature
\gset
\if :dont_have_feature
\quit
\endif

at the start of such regression tests. Then the alternative
'dont-have-the-feature' output file will stay the same when adding new
tests.

It's a bit annoying that when running such a sequence interactively one
can hit
if (pset.cur_cmd_interactive && !active_branch &&
!is_branching_command(cmd))
{
psql_error("\\%s command ignored; use \\endif or Ctrl-C to exit current \\if block\n",
cmd);
}
but given that's just when running interactively, it shouldn't be too
bad.

Greetings,

Andres Freund