[Lightx0x]

advance_epoch in tests/stake_program.rs reads the Clock, calls warp_to_slot,
then writes back the copy it read before the warp with epoch + 1. That
overwrites the warped slot, so the helper only advances the epoch:

	slot	epoch
before	459,648,000	0
after 1 call	459,648,000	1
after 2 calls	459,648,000	2

This change reads the Clock once, advances both slot (by slots_per_epoch)
and epoch, and writes it once. After the test's two calls the Clock is now
at slot 460,512,000, epoch 2.

The stake test passes before and after, since what it checks depends on the
epoch. This just makes the helper do what its name says, so slot and epoch
stay consistent.

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