Update SyslogAppenderTest to explicitly set maximum syslog message size (for test stability)

While trying to run the unit tests on my FreeBSD system, there was a failure:
```
java.lang.AssertionError: The string [<23>Nov 15 21:50:41 qk.local [main] ch.qos.logback.classic.net.SyslogAppenderTest hello] did not match regex [<\d{2}>\w{3} \d{2} \d{2}(:\d{2}){2} [\w.-]* \[main\] ch.qos.logback.classic.net.SyslogAppenderTest a{64000,66000}]
```

I found that it was due to FreeBSD's maximum UDP datagram size being 9216 bytes by default. The ```SyslogAppender``` uses a default of ```65000```, which works fine for Linux systems. 

It seemed prudent to utilize the SyslogAppender option to explicitly set the maximum size of a message and then update the test accordingly. This also becomes the first test to cover ```SyslogAppender.setMaxMessageSize()``` and it should make the test more stable for different environments.