Added sleep to hopefully reduce I/O test flakiness again

Author: Bungeefan

test/outputs/advanced_file_output_test.dart

@@ -120,18 +120,25 @@ void main() {
     output.output(event0);
     await output.destroy();
 
+    // Give the OS a chance to flush to the file system (should reduce flakiness)
+    await Future.delayed(const Duration(milliseconds: 50));
+
     // Start again to roll files on init without waiting for timer tick
     await output.init();
     final event1 = OutputEvent(LogEvent(Level.fatal, ""), ["2" * 1500]);
     output.output(event1);
     await output.destroy();
 
+    await Future.delayed(const Duration(milliseconds: 50));
+
     // And again for another roll
     await output.init();
     final event2 = OutputEvent(LogEvent(Level.fatal, ""), ["3" * 1500]);
     output.output(event2);
     await output.destroy();
 
+    await Future.delayed(const Duration(milliseconds: 50));
+
     final files = dir.listSync();
 
     // Expect only 2 files: the "latest" that is the current log file