Moved ancillary files to extras/ (fixes #1011)

extras/tests/Misc/custom_string.hpp

@@ -0,0 +1,14 @@
+// ArduinoJson - arduinojson.org
+// Copyright Benoit Blanchon 2014-2019
+// MIT License
+
+#pragma once
+
+#include <string>
+
+using namespace ARDUINOJSON_NAMESPACE;
+
+struct custom_char_traits : std::char_traits<char> {};
+struct custom_allocator : std::allocator<char> {};
+typedef std::basic_string<char, custom_char_traits, custom_allocator>
+    custom_string;