testsuite: skip tests that use COLUMNS if value not preserved

Problem: A couple tests in the testsuite explicitly set the COLUMNS
environment variable to ensure the output of a command responds to
changes in the terminal width. However, in some environments the
flux(1) libtool wrapper script can reset this variable, causing the
tests to erroneously fail.

Use the provided test_columns_variable_preserved() function to check
for the required behavior of this variable, and skip the tests if not.

Author: grondo

t/t1000-kvs.t

@@ -681,7 +681,8 @@ test_expect_success 'kvs: ls -w40 adjusts output width to 40' '
 	EOF
 	test_cmp expected output
 '
-test_expect_success 'kvs: ls with COLUMNS=20 adjusts output width to 20' '
+test_columns_variable_preserved && test_set_prereq USE_COLUMNS
+test_expect_success USE_COLUMNS 'kvs: ls with COLUMNS=20 adjusts output width to 20' '
 	flux kvs unlink -Rf $DIR &&
 	${FLUX_BUILD_DIR}/t/kvs/dtree -p$DIR -h1 -w50 &&
 	COLUMNS=20 flux kvs ls $DIR | wc -wl >output &&

t/t2803-flux-pstree.t

@@ -149,7 +149,8 @@ test_expect_success 'flux-pstree -a works' '
 	EOF
 	test_cmp ${name}.expected ${name}.output
 '
-test_expect_success 'flux-pstree truncates at COLUMNS' '
+test_columns_variable_preserved && test_set_prereq USE_COLUMNS
+test_expect_success USE_COLUMNS 'flux-pstree truncates at COLUMNS' '
 	name="truncated" &&
 	COLUMNS=16 flux pstree -a > ${name}.output &&
 	test_debug "cat ${name}.output" &&