Add a trivial callback sink (#2610)

Add a trivial callback sink

tests/CMakeLists.txt

@@ -31,6 +31,7 @@ set(SPDLOG_UTESTS_SOURCES
     test_stdout_api.cpp
     test_backtrace.cpp
     test_create_dir.cpp
+    test_custom_callbacks.cpp
     test_cfg.cpp
     test_time_point.cpp
     test_stopwatch.cpp)