test-lib.sh: set COLUMNS=80 for --verbose repeatability

Some tests will fail under --verbose because while we've unset COLUMNS
since b1d645b (tests: unset COLUMNS inherited from environment,
2012-03-27), we also look for the columns with an ioctl(..,
TIOCGWINSZ, ...) on some platforms. By setting COLUMNS again we
preempt the TIOCGWINSZ lookup in pager.c's term_columns(), it'll take
COLUMNS over TIOCGWINSZ,

This fixes t0500-progress-display.sh., which broke because of a
combination of the this issue and the progress output reacting to the
column width since 545dc34 (progress: break too long progress bar
lines, 2019-04-12). The t5324-split-commit-graph.sh fails in a similar
manner due to progress output, see [1] for details.

The issue is not specific to progress.c, the diff code also checks
COLUMNS and some of its tests can be made to fail in a similar
manner[2], anything that invokes a pager is potentially affected.

See ea77e67 (Make "git help" react to window size correctly,
2005-12-18) and ad6c373 (pager: find out the terminal width before
spawning the pager, 2012-02-12) for how the TIOCGWINSZ code ended up
in pager.c

1. http://lore.kernel.org/git/20210624051253.GG6312@szeder.dev
2. https://lore.kernel.org/git/20210627074419.GH6312@szeder.dev/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

t/test-lib.sh

@@ -401,14 +401,15 @@ LANG=C
 LC_ALL=C
 PAGER=cat
 TZ=UTC
-export LANG LC_ALL PAGER TZ
+COLUMNS=80
+export LANG LC_ALL PAGER TZ COLUMNS
 EDITOR=:
 
 # A call to "unset" with no arguments causes at least Solaris 10
 # /usr/xpg4/bin/sh and /bin/ksh to bail out.  So keep the unsets
 # deriving from the command substitution clustered with the other
 # ones.
-unset VISUAL EMAIL LANGUAGE COLUMNS $("$PERL_PATH" -e '
+unset VISUAL EMAIL LANGUAGE $("$PERL_PATH" -e '
 	my @env = keys %ENV;
 	my $ok = join("|", qw(
 		TRACE