mgca: Add test for normalizing const projections with assoc const equality bounds

[fmease]

EDIT(@camelid): If you're interested in doing this, please add a //@ check-pass test to tests/ui/const-generics/mgca/ that is along the lines of the example in #120905 (comment). Feel free to ping me (@camelid) or assign me on your PR.

---

For example, I expected the following code to succeed compilation:

#![feature(generic_const_exprs)]
#![feature(associated_const_equality)]

trait Trait {
    const C: usize;
}

fn f<T: Trait<C = 1>>()
where
    [(); T::C]:
{
    let _: [(); T::C] = [()];
}

Since (I'm pretty sure that) the ParamEnv contains the projection predicate <T as Trait>::C == 1, the const projection <T as Trait>::C inside the function body should be able to get normalized to 1. That however doesn't seem to happen:

error[E0308]: mismatched types
  --> src/lib.rs:12:25
   |
12 |     let _: [(); T::C] = [()];
   |                         ^^^^ expected `T::C`, found `1`
   |
   = note: expected constant `T::C`
              found constant `1`

[fmease]

TODO: Find a reproducer that isn't reliant on GCE. Reliance on mGCE/mGCA (min_generic_const_args) is fine on the other hand (once that's implemented).