testsuite: test for shell output truncation in t9000-system.t

Problem: There is no test that ensures the job shell truncates output
to 10M by default for guest jobs in a multiuser instance.

Add a test to t9000-system.t that ensures output is truncated in this
case.

t/system/0001-basic.t

@@ -15,3 +15,9 @@ test_expect_success 'flux jobs lists job with correct userid' '
 test_expect_success 'flux proxy can submit jobs to system instance' '
 	flux proxy $(flux getattr local-uri) flux submit true
 '
+test_expect_success 'flux-shell limits kvs output to 10M for guest jobs' '
+	dd if=/dev/urandom bs=10240 count=800 | base64 --wrap 79 >trunc.exp &&
+	flux run cat expected >output 2>trunc.err &&
+	test_debug "cat trunc.err" &&
+	grep "stdout.*truncated" trunc.error
+'