Confusion about String handling
The {0} denotes an extra parameter (which you want to print), it seems you just want to print the string without any variable 'value' so you can remove the {0}.

I'm not so familiar with using {0} in C++, but in C# {0} means argument number 0 (after the initial string), since this argument is not given, probably an empty string is created ... it would be more intuitive if the compiler give an error or warning (maybe it gives a warning and you can check for it).

Also converting a string to String() does not do anything but I think it was just for debugging/checking purposes.

Also in Arduino to save SRAM space, normally it is wise to add before each string, the F function like:

Serial.println(F("Some string to print"));


This will create a memory in Flash and will not be copied to SRAM which is normally quite scarce.

To print out arguments you can simply use multiple print commands (which does not add a new line) and the last part of the line with println, like:

Serial.print(F("Test time: ");
Serial.println(test_time);