¿Por qué en el mundo falla la siguiente prueba? (está en xunit) Lo he probado con diferentes agregadores y nunca escribe nada, aunque el registro parece estar listo para escribir. Eventualmente creé mi propio appender solo para probarlo.

    public class TestAppender : AppenderSkeleton {
        public event Action<LoggingEvent> AppendCalled = delegate { };
        protected override void Append(LoggingEvent loggingEvent) {
            AppendCalled(loggingEvent);
        }
    }
    public class Class1 {
        private TestAppender _appender = new TestAppender();
        public Class1() {
            log4net.Util.LogLog.InternalDebugging = true;
            Hierarchy hierarchy = (Hierarchy)LogManager.GetRepository();
            Logger rootLogger = hierarchy.Root;
            rootLogger.Level = Level.All;
            Logger coreLogger = hierarchy.GetLogger("abc") as Logger;
            coreLogger.Level = Level.All;

            coreLogger.Parent = rootLogger;
            PatternLayout patternLayout = new PatternLayout();
            patternLayout.ConversionPattern = "%logger - %message %newline";
            patternLayout.ActivateOptions();
            _appender.Layout = patternLayout;
            _appender.ActivateOptions();
            coreLogger.AddAppender(_appender);            
        }
        [Fact]
        public void Test() {
            bool called = false;
            _appender.AppendCalled += e => called = true;
            var log = LogManager.GetLogger("abc");
            log.Debug("This is a debugging message");
            Thread.Sleep(TimeSpan.FromSeconds(2));
            log.Info("This is an info message");
            Thread.Sleep(TimeSpan.FromSeconds(2));
            log.Warn("This is a warning message");
            Thread.Sleep(TimeSpan.FromSeconds(2));
            log.Error("This is an error message");
            Assert.True(called);
        }
}