Added FileOutput test

Author: Bungeefan

test/outputs/file_output_test.dart

@@ -0,0 +1,40 @@
+import 'dart:io';
+
+import 'package:logger/logger.dart';
+import 'package:test/test.dart';
+
+void main() {
+  var file = File("${Directory.systemTemp.path}/dart_logger_test.log");
+  setUp(() async {
+    await file.create(recursive: true);
+  });
+
+  tearDown(() async {
+    await file.delete();
+  });
+
+  test('Real file read and write', () async {
+    var output = FileOutput(file: file);
+    await output.init();
+
+    final event0 = OutputEvent(LogEvent(Level.info, null), ["First event"]);
+    final event1 = OutputEvent(LogEvent(Level.info, null), ["Second event"]);
+    final event2 = OutputEvent(LogEvent(Level.info, null), ["Third event"]);
+
+    output.output(event0);
+    output.output(event1);
+    output.output(event2);
+
+    await output.destroy();
+
+    var content = await file.readAsString();
+    expect(
+      content,
+      allOf(
+        contains("First event"),
+        contains("Second event"),
+        contains("Third event"),
+      ),
+    );
+  });
+}