Add test to demonstrate failed case with ToCppString

[Choochmeque]

Test fails with:

test /var/folders/t0/6220z7_s6mzcvxrdk80tbv640000gn/T/.tmp7cfHIg/input.rs ... error
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
error[E0405]: cannot find trait `ToCppString` in this scope
 --> /var/folders/t0/6220z7_s6mzcvxrdk80tbv640000gn/T/.tmptOO7lq/autocxx-ffi-default-gen.rs:1:5299
  |
   1 | ... { cxxbridge , output , bindgen } ; pub fn measure_string (z : impl ToCppString) -> u32 { cxxbridge :: measure_string_autocxx_wrapper_0x23632f402293...
     |                                                                        ^^^^^^^^^^^
     |
    ::: $RUST/alloc/src/string.rs
     |
     | pub trait ToString {
     | ------------------ similarly named trait `ToString` defined here
     |
help: a trait with a similar name exists

It's a good idea to open an issue first for discussion.

• Tests pass
• Appropriate changes to README are included in PR