Add test for RPIT defined with different hidden types with different substs

[BoxyUwU]

The following code:

trait Trait {}
impl Trait for () {}

fn foo<T: Trait, U: Trait>() -> impl Trait {
    let a: T = foo::<T, U>();
    loop {}
    let _: T = foo::<U, T>();
}

used to incorrectly compile but now correctly errors, we should add a test to ensure this doesn't regress.

cc #111853 @compiler-errors

[obeis]

@rustbot claim