support/testing: add test for syslog-ng

support/testing: add test for syslog-ng

Check:
 - the daemon is started;
 - a message is collected;
 - the daemon does not issue a warning message on startup.

When the .conf file version does not match the package version a warning
message shows up on serial on every boot. This message is generated by
syslog-ng before it is running, so it is not logged to
/var/log/messages. So in order to test the message is generated, restart
the server. It makes the message appears on /var/log/messages (since the
server is already running) where its existence can be easily tested
using grep.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@datacom.ind.br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>