testsuite: update test message

Problem: A test in t2800-jobs-cmd.t had a message that was inconsistent
to what the test performed.

Update the test message to list what is tested.  Update the format of the
message to avoid it being super long.

Author: chu11

t/t2800-jobs-cmd.t

@@ -470,7 +470,7 @@ test_expect_success 'flux-jobs --format={id} works' '
 	test_cmp idsI.out inactive.ids
 '
 
-test_expect_success 'flux-jobs --format={id.f58},{id.hex},{id.dothex},{id.words} works' '
+test_expect_success 'flux-jobs --format={id.dec/f58/hex/kvs/dothex/words} works' '
 	flux jobs -ano {id.dec},{id.f58},{id.hex},{id.kvs},{id.dothex},{id.words} \
 	    | sort -n > ids.XX.out &&
 	for id in $(cat all.ids); do