test-lib: skip test with COLUMNS=1 under mksh

test-lib: skip test with COLUMNS=1 under mksh

mksh does not allow $COLUMNS to be set below 12.  mksh(1) says that
$COLUMNS is "always set, defaults to 80, unless the value as reported
by stty(1) is non-zero and sane enough". This applies also to setting
it directly for one command:

    $ COLUMNS=10 python -c 'import os; print os.environ["COLUMNS"]'
    98

Add a test prerequisite by checking if we can set COLUMNS=1, to allow
us to skip tests that needs it.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>