Tezlink/Kernel/Test: Fix unit test dependency for tezos-tezlink-latest crate

What

This MR fixes a missing dependency issue when running unit tests for the tezos_tezlink_latest crate in isolation.

Why

Running cargo test directly within the tezos_tezlink_latest crate fails due to a missing implementation of H256::random(), which requires the rand feature of the fixed-hash crate. While the full project compiles fine with the make -f etherlink.mk flow, isolated testing in IDEs or via cargo test breaks.

How

• Keep the H256::random() call in tests but add the optional rand feature during test compilation.

Manually testing the MR

To verify the fix:

1. Run cargo test inside the etherlink/kernel_latest/tezos/ crate directly.
2. Ensure that the test dummy_applied_operation compiles and executes correctly.
3. Run make -f etherlink.mk to confirm the global build is unaffected.

cd etherlink/kernel_latest/tezos/
cargo test