Add integration test utilities to create multiple accounts with balance.

[LuisPH3]

This is a small contribution from uncommitted branches, I have seen the pattern recently in tests and this can come out handy.
The function is located next to MakeAccountWithBalance to avoid fragmentation.

[Copilot]

Pull request overview

This PR introduces a new integration-test utility to create multiple funded accounts (with accessible private keys) and refactors existing tests to use it, plus adds direct tests for the new helper.

Changes:

• Add MakeAccountsWithBalance helper alongside MakeAccountWithBalance.
• Add integration tests validating MakeAccountWithBalance and MakeAccountsWithBalance.
• Refactor existing protocol/load tests to use the new helper.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
tests/test_utils.go	Adds MakeAccountsWithBalance helper and related logic.
tests/test_utils_test.go	Adds new integration tests covering the account-funding utilities.
tests/single_proposer/single_proposer_protocol_test.go	Refactors account creation/endowment to use the new helper.
tests/large_transactions/large_transactions_test.go	Refactors account creation/endowment to use the new helper (but changes funding amount).

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[facuMH]

Overall looks good. I just have a couple of questions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 10 files with indirect coverage changes

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[facuMH]

Thanks for considering changes. Looks good.
If more changes are requested later consider making long require lines into multi-lines, but it is only a minor style remark since this is only test tooling.