If you wanted to write a drop-in SQLite replacement in Rust, using the original project's (incredible) test suite would be on the mandatory side, wouldn't it?

To a degree. Certainly the end-to-end tests should be something you can expect to reuse. Other tests may be hit or miss. In the case of SQLite, where there's >100x as much test code as application code, there's a large range between "can use some of it" and "can use most of it" - as mathw gets at I think we should be able to expect the latter here.

Definitely! And Rust is capable of that kind of thing (producing a drop-in replacement for a C library) so there'd be no excuses either.