added async tests

tests/test_sink.h

+//
+// Copyright(c) 2018 Gabi Melman.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+//
+
+#pragma once
+
+#include "spdlog/details/null_mutex.h"
+#include "spdlog/sinks/base_sink.h"
+
+#include <mutex>
+
+namespace spdlog {
+namespace sinks {
+
+template<class Mutex>
+class test_sink : public base_sink<Mutex>
+{
+public:
+    size_t msg_counter()
+    {
+        return msg_counter_;
+    }
+
+    size_t flushed_msg_counter()
+    {
+        return flushed_msg_counter_;
+    }
+
+protected:
+    void _sink_it(const details::log_msg &) override
+    {
+        msg_counter_++;
+    }
+
+    void _flush() override
+    {
+        flushed_msg_counter_ += msg_counter_;
+    }
+    size_t msg_counter_{0};
+    size_t flushed_msg_counter_{0};
+};
+
+using test_sink_mt = test_sink<std::mutex>;
+using test_sink_st = test_sink<details::null_mutex>;
+
+} // namespace sinks
+} // namespace spdlog