1. Fixed file_helper::exists() bug under windows which returned false in some circumstances

2. Improved file_helper::exists() performance under linux to use stat sys call
3. Added unit tests

tests/includes.h

@@ -10,3 +10,14 @@
 #include "catch.hpp"
 #include "../include/spdlog/spdlog.h"
 #include "../include/spdlog/sinks/null_sink.h"
+
+
+static void prepare_logdir()
+{
+    spdlog::drop_all();
+#ifdef _WIN32
+    auto rv = system("del /F /Q logs\\*");
+#else
+    auto rv = system("rm -f logs/*");
+#endif
+}