testsuite: set system test certificate name

Problem: system instance tests do not set the certificate name,
but this will be the recommended practice.

Change flux-sharness.sh to set a certificate name of "testcert",
then check that it appears in the "overlay auth" log messages in
one of the system tests.

Author: garlick

t/sharness.d/flux-sharness.sh

@@ -99,7 +99,7 @@ make_bootstrap_config() {
 
     mkdir $workdir/conf.d
     mkdir $workdir/state
-    flux keygen $workdir/cert
+    flux keygen --name testcert $workdir/cert
     cat >$workdir/conf.d/bootstrap.toml <<-EOT
 	[bootstrap]
 	    curve_cert = "$workdir/cert"

t/t3300-system-basic.t

@@ -31,6 +31,10 @@ test_expect_success 'startctl status works' '
 test_expect_success HAVE_JQ 'broker overlay shows 2 connected children' '
 	test $(overlay_connected_children) -eq 2
 '
+test_expect_success 'testcert was used to authenticate' '
+	flux dmesg |grep "overlay auth" >auth.log &&
+	grep testcert auth.log
+'
 
 test_expect_success 'overlay status is full' '
 	test "$(flux overlay status --timeout=0 --summary)" = "full"