If the header file is included test_utils_interactive_sync() will
be defined as an empty function when the module is not used, e.g.
added to DISABLE_MODULES in tests/*
Add an implementation that waits for 's' to print 'START' and return.
If 'r' is given is prints 'READY' to allow querying for state.
The help and answered string have to be different to not match the other.
Using puts/getchar was smaller than using `stdio_read/stdio_write` on the
example I tested with `esp32`.
