W/o this change, file::write('a'); tries to use the template and fails
since char is a basic type.
The reason it is needed is due to pre 2.5.x behavior, File::write(char)
silently was cast to File::write(uint8_t). With the template write,
though, this is not performed.
* Add Print::write tests and add'l overrides
Ensure that print::write does something sane and doesn't cause a compile
time error about templates when used for Files.
Test using SPIFFS file since Print is an abstract type.
