You can get ConsoleAppender from logger repository and change it's layout:
BasicConfigurator.Configure();
var appender = LogManager.GetRepository()
.GetAppenders()
.OfType<ConsoleAppender>()
.First();
appender.Layout = new PatternLayout("%d %-5level %logger - %m%n"); // set pattern
ILog logger = LogManager.GetLogger(logger_name); // obtain logger
But configuration via config file is preferred way, because you can change settings without recompiling your tests.