test: use bootstrap_account type when possible when creating a new context

The function Account.generation_accounts used to return a list of tuples that must be used in block.genesis. The function took a balance and bootstrap_delegation as optional parameters but did nothing with it. So instead I split that function in two generate_accounts that creates accounts and make_bootstrap_accounts.

I also remove some dead code and used the record Default_parameters.bootstrap_account when it make sense instead of a tuple.

Context

Manually testing the MR

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