test(nat): Remove warning for shuttle tests

We have a warning when building the shuttle tests for nat:

    $ cargo check --features shuttle --all-targets -p dataplane-nat
    [...]
    warning: function `vpcd1` is never used
      --> nat/src/masquerade/apalloc/test_alloc.rs:43:12
       |
    43 |     pub fn vpcd1() -> VpcDiscriminant {
       |            ^^^^^
       |
       = note: `#[warn(dead_code)]` (part of `#[warn(unused)]`) on by default

It's not blocking, but let's suppress it anyway.

Signed-off-by: Quentin Monnet <qmo@qmon.net>

Author: qmonnet

nat/src/masquerade/apalloc/test_alloc.rs

@@ -40,6 +40,7 @@ mod context {
     pub fn vni2() -> Vni {
         Vni::new_checked(200).unwrap()
     }
+    #[allow(unused)]
     pub fn vpcd1() -> VpcDiscriminant {
         VpcDiscriminant::from_vni(vni1())
     }