MDEV-7330 plugins.feedback_plugin_send fails sporadically in buildbot

The test restarts the server and expects that the feedback plugin
will send a report on shutdown, and will write about it in the error
log. But the server is only given 10 sec to shut down properly,
which is not always enough.
Added a parameter to restart_mysqld.inc, and set it to a bigger
value in feedback_plugin_send

mysql-test/include/restart_mysqld.inc

@@ -1,3 +1,8 @@
+# ==== Usage ====
+#
+# [--let $shutdown_timeout= 30]
+# [--let $allow_rpl_inited= 1]
+# --source include/restart_mysqld.inc
 
 if ($rpl_inited)
 {
@@ -7,6 +12,16 @@ if ($rpl_inited)
   }
 }
 
+--let $server_shutdown_timeout= 10
+if ($shutdown_timeout) 
+{
+  --let $server_shutdown_timeout= $shutdown_timeout
+}
+if ($shutdown_timeout == 0)
+{
+  --let $server_shutdown_timeout= 0
+}
+
 # Write file to make mysql-test-run.pl expect the "crash", but don't start
 # it until it's told to
 --let $_server_id= `SELECT @@server_id`
@@ -15,7 +30,7 @@ if ($rpl_inited)
 
 # Send shutdown to the connected server and give
 # it 10 seconds to die before zapping it
-shutdown_server 10;
+shutdown_server $server_shutdown_timeout;
 
 # Write file to make mysql-test-run.pl start up the server again
 --exec echo "restart" > $_expect_file_name

mysql-test/suite/plugins/t/feedback_plugin_send.test

@@ -14,6 +14,15 @@ if (!$MTR_FEEDBACK_PLUGIN) {
 # is doing some work in other workers.
 #
 sleep 310;
+
+# The test expects that the plugin will send a report at least 2 times,
+# now (5 min after loading) and on server shutdown which happens below. 
+# Since we have already waited for 5 min, let's be generous
+# and make sure the server has enough time to shut down properly.
+# We won't lose anything if the shutdown is fast, but if it's slow, the plugin 
+# will still be able to finish the job and write about it in the error log.
+
+--let $shutdown_timeout= 60
 source include/restart_mysqld.inc;
 
 replace_result https http;