testsuite: test shell output truncation

Problem: The job shell should truncate standard output at 10M, but
there is no test for this in the testsuite.

Add tests that stdout and stderr are truncated at this size. Use
the LONGTEST prereq since this test takes some time.

Author: grondo

t/t2606-job-shell-output-redirection.t

@@ -348,4 +348,15 @@ test_expect_success 'job-shell: shell errors are captured in error file' '
 	test_expect_code 127  flux run --error=test.err nosuchcommand &&
 	grep "nosuchcommand: No such file or directory" test.err
 '
+test_expect_success LONGTEST 'job-shell: output to kvs is truncated at 10MB' '
+	dd if=/dev/urandom bs=10240 count=800 | base64 --wrap 79 >expected &&
+	flux run cat expected >output 2>truncate.error &&
+	test_debug "cat truncate.error" &&
+	grep "stdout.*truncated" truncate.error
+'
+test_expect_success LONGTEST 'job-shell: stderr to kvs is truncated at 10MB' '
+	dd if=/dev/urandom bs=10240 count=800 | base64 --wrap 79 >expected &&
+	flux run sh -c "cat expected >&2"  >truncate2.error 2>&1 &&
+	grep "stderr.*truncated" truncate2.error
+'
 test_done