Add a simple test for the new constructor

Convert a c-str to a FPSTR in the example to have a section of code in our CI that will catch any future breaks of this specific kind.
esp8266 · earlephilhower · Feb 23, 2020 · Feb 23, 2020 · Feb 23, 2020 · Feb 23, 2020
commit e9b4fbae4a2294e4149702a968e0cee3f146a629
diff --git a/libraries/ESP8266WebServer/examples/PathArgServer/PathArgServer.ino b/libraries/ESP8266WebServer/examples/PathArgServer/PathArgServer.ino
@@ -37,7 +37,7 @@ void setup(void) {
     Serial.println("MDNS responder started");
   }
 
-  server.on("/", []() {
+  server.on(F("/"), []() {
     server.send(200, "text/plain", "hello from esp8266!");
   });