[TKN-714] add unit tests to validate anchor account discriminators

[jshiohaha]

Description

• Anchor >=0.31 no longer enforces an 8-byte discriminator. These tests ensure that we do not accidentally introduce a change that impacts existing account discriminators.

Testing

• Build: anchor build -p whirlpool
• Unit tests: cargo test -p whirlpool --lib

[yugure-orca]

almost LGTM, but could you check the followings ?

• WhirlpoolsConfig discriminator value in code comment.
• how about using hex number because it is easy to check (no hex to dec conversion)
• DynamicTickArray account also has discriminator: https://github.com/orca-so/whirlpools/blob/main/programs/whirlpool/src/state/dynamic_tick_array.rs#L106

[jshiohaha]

almost LGTM, but could you check the followings ?

• WhirlpoolsConfig discriminator value in code comment.
• how about using hex number because it is easy to check (no hex to dec conversion)
• DynamicTickArray account also has discriminator: https://github.com/orca-so/whirlpools/blob/main/programs/whirlpool/src/state/dynamic_tick_array.rs#L106

Thanks for the comments! I replied to most comments directly, but for the last 1 here - I actually based all my tests' name and content on the existing dynamic tick array discriminator test, so I think that case is covered - unless I'm missing smth else 👀

[yugure-orca]

Thansk for the update!
(sorry I missed the existing test case for DynamicTickArray)

LGTM with 1 super nit.