Merge branch 'zj/mksh-columns-breakage'

A broken shell may not let us set an environment value to an arbitrary
value, interfering with some of the tests. Introduce a test prerequisite
so that we can skip some tests on such a platform.

By Zbigniew Jędrzejewski-Szmek
* zj/mksh-columns-breakage:
  test-lib: skip test with COLUMNS=1 under mksh

diff --cc t/test-lib.sh
Simple merge