solana-program-test: warp_to_slot resets program cache, causing slowness

[ckamm]

Problem

For the mango program we recently investigated upgrading to solana 1.16.14. When we did, the program tests started taking >10x as long.

Debugging showed that every single transaction we sent with banks_client.process_transaction_with_commitment() would take 280ms in Bank::replenish_program_cache to reload the mango and token programs.

This happened because we used ProgramTestContext::warp_to_slot after every transaction. Apparently that used to preserve the cache but now resets it somehow.

We were able to work around the issue by removing warp_to_slot. (we only cared about new blockhashes for every transaction, and using the new get_new_latest_blockhash() works just as well for that)

Proposed Solution

Make warp_to_slot() retain the program cache as before