Update soroban settings files and utils

[sisuresh]

Description

1. Add a testnet_settings.json file for testnet copied from pubnet_phase.json.
2. Update testnet to use bucket_list_target_size_bytes=200000000 and write_fee1_kb_bucket_list_low=-12034.
3. Update utils to allow for lower target sizes.

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[dmkozh]

Shall we also update the target BL size for pubnet (to 13000000000)? We'll need to double-check it, but it seems unlikely that BL size would change significantly before phase 1.

[sisuresh]

Ah yeah good catch. Fixed

[dmkozh]

r+ 8b1d8f1