[Lightx0x]

Adds tests for warp_to_slot's documented behaviour. warp_to_slot sets Clock.slot and leaves the rest of the Clock unchanged. One test warps forward, one backward, and both compare the whole Clock against the one read before the warp.

Five existing tests call warp_to_slot, and two would catch it not setting the slot, but none check the other fields. A version that also changes the epoch passes them all. Mutation check, with clock.epoch += 1 added inside
warp_to_slot:

test	result
compute_budget	9 passed
stake_program	1 passed
persistence roundtrip	20 passed
warp_to_slot (this PR)	2 failed

Tested: cargo test --features precompiles, all passed; fmt --check and clippy --all-features --all-targets clean.

Related: #451, which fixes a test helper that undid its own warp.