made shutdown timeout configurable, included integration tests and unit tests

Author: Amberley-Sz

tracing-appender/src/non_blocking.rs

@@ -383,7 +383,7 @@ impl ErrorCounter {
 #[cfg(test)]
 mod test {
     use super::*;
-    use std::sync::{mpsc, Mutex};
+    use std::sync::mpsc;
     use std::thread;
     use std::time::Duration;