testsuite: improve flux resource eventlog -F test

Problem: The test for `flux resource eventlog -F` doesn't check that
the command actually waited for the drain event. There is also a
race between launching the command in the background and draining a
rank where the resource.journal RPC could come after the drain RPC,
thus the command doesn't actually have to follow the eventlog.

Ensure that the RPC is established before moving on to drain a rank
by waiting for output to appear. Check that the drain event is the
final event in the output file.

Author: grondo

t/t2355-resource-journal.t

@@ -119,16 +119,21 @@ test_expect_success 'that event WAS posted to the KVS' '
 	test $(grep resource-define kvs2 | wc -l) -eq 2
 '
 test_expect_success NO_CHAIN_LINT 'watch eventlog in the background waiting on drain' '
-	flux resource eventlog -F --wait=drain >bgeventlog &
-	echo $! >bgpid
+	flux resource eventlog -HF --wait=drain >bgeventlog &
+	echo $! >bgpid &&
+	# ensure RPC is established before moving on
+	waitfile.lua -vt 15 bgeventlog
 '
 test_expect_success NO_CHAIN_LINT 'drain rank 1' '
 	flux resource drain 1
 '
 test_expect_success NO_CHAIN_LINT 'background watcher completed successfully' '
 	wait $(cat bgpid)
 '
-
+test_expect_success NO_CHAIN_LINT 'background watcher waited for drain event' '
+	test_debug "cat bgeventlog" &&
+	tail -1 bgeventlog | grep drain
+'
 test_expect_success 'run restartable flux instance, drain 0' '
 	flux start --setattr=statedir=$(pwd) \
 	    sh -c "flux resource eventlog --wait=resource-define \