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

t/test-lib.sh

@@ -615,6 +615,7 @@ case $(uname -s) in
 	;;
 esac
 
+( COLUMNS=1 && test $COLUMNS = 1 ) && test_set_prereq COLUMNS_CAN_BE_1
 test -z "$NO_PERL" && test_set_prereq PERL
 test -z "$NO_PYTHON" && test_set_prereq PYTHON
 test -n "$USE_LIBPCRE" && test_set_prereq LIBPCRE