testsuite: test flux-watch(1) with binary output

Problem: Handling of binary output with flux-watch(1) is not tests.

Add a test that ensures flux-watch(1) can handle binary data
end-to-end.

Author: grondo

t/t2813-flux-watch.t

@@ -130,4 +130,11 @@ test_expect_success 'flux-watch: --filter works' '
 	test_debug "cat failed.out" &&
 	grep "Watching ${nfailed} job" failed.out
 '
+test_expect_success 'flux-watch: handles binary data' '
+	id=$(flux submit dd if=/dev/urandom count=1) &&
+	test_debug "flux job eventlog -p guest.output -HL $id" &&
+	flux job attach $id >binary.expected &&
+	flux watch $id >binary.output &&
+	test_cmp binary.expected binary.output
+'
 test_done