testsuite: ensure valid UTF-8 can be logged in dmesg

Problem: None of the tests in t0009-dmesg.t ensure that valid utf-8,
non-ascii characters can be logged in the broker's dmesg buffer.

Add a test that ensures a valid UTF-8 string can be logged successfully
and is displayed properly with `flux-dmesg(1)`.

Author: grondo

t/t0009-dmesg.t

@@ -108,6 +108,12 @@ test_expect_success 'logged non-ascii characters handled ok' '
 	/bin/echo -n -e "\xFF\xFE\x82\x00" | flux logger &&
 	flux dmesg
 '
+test_expect_success 'logged non-ascii printable characters are unmodified' '
+	flux logger ƒ Φ Ψ Ω Ö &&
+	flux dmesg | tail -1 > dmesg.utf-8 &&
+	test_debug "cat dmesg.utf-8" &&
+	grep "ƒ Φ Ψ Ω Ö" dmesg.utf-8
+'
 test_expect_success 'dmesg request with empty payload fails with EPROTO(71)' '
 	${RPC} log.dmesg 71 </dev/null
 '