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