Add std::tm formatter, fix spdlog::stopwatch formatter, conditionally use fmt::runtime in test_errors

include/spdlog/stopwatch.h

@@ -48,7 +48,14 @@ public:
 } // namespace spdlog
 
 // Support for fmt formatting  (e.g. "{:012.9}" or just "{}")
-namespace fmt {
+namespace
+#ifdef SPDLOG_USE_STD_FORMAT
+    std
+#else
+    fmt
+#endif
+{
+
 template<>
 struct formatter<spdlog::stopwatch> : formatter<double>
 {
@@ -58,4 +65,4 @@ struct formatter<spdlog::stopwatch> : formatter<double>
         return formatter<double>::format(sw.elapsed().count(), ctx);
     }
 };
-} // namespace fmt
+} // namespace fmt/std