cleanup tests & test support code

[dpaiton]

Description

• I moved a validity check from calculate_open_short to solvency_after_short. And we now call solvency_after_short in calculate_open_short. This diverges a bit from Solidity because we do not split the apply_open_short function from the calculate_open_short function in rust. This means it was possible to get a valid answer from calculate_open_short that would revert in the contract. That should be fixed, now.
• I updated a bunch of tests to check if a short is possible by directly calling solvency_after_short(min_txn) instead of the incorrect check it was doing.
• I removed the get_max_short function from the preamble. We don't need all of the extra checks.
• I modified the preamble random state generator to be faster by taking fewer steps and using the approximate max short. This was a really slow step in the solidity tests.
• the Agent class now uses the new functions when you call agent.calculate_max_short
• I increased the defaults for calculate_short_bonds_given_deposit to maybe_base_tolerance=1e10 (from 1e9) and maybe_max_iterations=1_000 (from 500).