[DA] Planetscale engine tests: one2m_mix_required_writable_readable

[miguelff]

This is a subcluster of failures after having enabled planetscale tests in #4423

Failing tests in this cluster:

writes::relations::compound_fks_mixed_requiredness::compound_fks::one2m_mix_required_writable_readable

Please follow this process

[jkomyno]

I can confirm that this still fails on both Napi and Wasm, with

Expected result to return an error, but found success: {"data":{"createOnePost":{"id":2,"user_id":2,"user_age":2,"User":null}}}

The reason is that, on PlanetScale, we don't have foreign key violations. We should thus get rid of

  // Foreign key violation
  assert_error!(
      &runner,
      r#"mutation{createOnePost(data:{id: 2, user_id:2, user_age: 2}){id, user_id, user_age, User{id}}}"#,
      2003
  );

for PlanetScale only.

[jkomyno]

Fixed via #4909