Moved ancillary files to extras/ (fixes #1011)

extras/tests/MixedConfiguration/enable_nan_0.cpp

@@ -0,0 +1,25 @@
+#define ARDUINOJSON_ENABLE_NAN 0
+#include <ArduinoJson.h>
+
+#include <catch.hpp>
+#include <limits>
+
+TEST_CASE("ARDUINOJSON_ENABLE_NAN == 0") {
+  DynamicJsonDocument doc(4096);
+  JsonObject root = doc.to<JsonObject>();
+
+  SECTION("serializeJson()") {
+    root["X"] = std::numeric_limits<double>::signaling_NaN();
+
+    std::string json;
+    serializeJson(doc, json);
+
+    REQUIRE(json == "{\"X\":null}");
+  }
+
+  SECTION("deserializeJson()") {
+    auto err = deserializeJson(doc, "{\"X\":NaN}");
+
+    REQUIRE(err == DeserializationError::InvalidInput);
+  }
+}