feat(testutil/validatormock): port synccomm.go (SyncCommMember)

Ports `charon/testutil/validatormock/synccomm.go` to Rust. SyncCommMember
drives PrepareEpoch → PrepareSlot → Message → Aggregate; per-slot ready flags
use Arc<tokio::sync::OnceCell<()>> to mirror Go's lazy `chan struct{}` maps.

TestGetSubcommittees ports the Go internal test verbatim.

Author: varex83

crates/testutil/src/validatormock/mod.rs

@@ -9,10 +9,12 @@ pub mod capture;
 pub mod error;
 pub mod meta;
 pub mod sign;
+pub mod synccomm;
 pub mod validators;
 
 pub use capture::{EndpointMatch, SubmissionCapture};
 pub use error::{Error, Result, SignError};
 pub use meta::{MetaEpoch, MetaSlot, SpecMeta};
 pub use sign::{Sign, SignFunc, Signer};
+pub use synccomm::{SyncCommMember, SyncCommitteeDuty};
 pub use validators::{ActiveValidators, active_validators};