fix: Ignore unsupported test in miri (#794)

The miri test in CI currently
[fails](https://github.com/CQCL/hugr/actions/runs/7447240566/job/20259152793)
with an error `"unknown variant 'ConstF64', there are no variants"`.

This is due to `CustomConst` serialization depending on `typetag`, which
registers each constant variant at the start of the program using `ctor`
(i.e. running code before `main`).

Miri does not currenly [support running
this](https://www.github.com/rust-lang/miri/issues/450), so the
deserialization finds the CustomConst registry empty (hence the "there
are no variants").

The solution until support is added to miri is just to ignore test with
constants.

src/hugr/serialize.rs

@@ -483,6 +483,9 @@ pub mod test {
     }
 
     #[test]
+    #[cfg_attr(miri, ignore = "Extension ops cannot be used with miri.")]
+    // Miri doesn't run the extension registration required by `typetag` for registering `CustomConst`s.
+    // https://github.com/rust-lang/miri/issues/450
     fn constants_roundtrip() -> Result<(), Box<dyn std::error::Error>> {
         let mut builder = DFGBuilder::new(FunctionType::new(vec![], vec![FLOAT64_TYPE])).unwrap();
         let w = builder.add_load_const(ConstF64::new(0.5))?;