Add some tests

[a-rodin]

This PR adds some tests for the codegen part of the code.

[a-rodin]

@Handy-caT using #[derive(PersistTable)] on a structure results in a compilation error:

error: proc-macro derive panicked
  --> tests/mod.rs:31:10
   |
31 | #[derive(PersistTable)]
   |          ^^^^^^^^^^^^
   |
   = help: message: called `Option::unwrap()` on a `None` value

[Handy-caT]

@a-rodin It's because PersistTable macro is related to wt only. I think this macro even must be moved into wt repo