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>