[cluster-test] Fix mint_account when num_accounts % num_instances != 0
[cluster-test] Fix mint_account when num_accounts % num_instances != 0

There was a bug in mint_account that did not calculate properly number of accounts that needs to be minted

When requested to mint small number of accounts(say 15), it would mint 0 because `num_new_accounts` was incorrectly dividing num_accounts / num_instances.

It would also fail basically on any number of accounts that does not evenly divide by `req.instances.len()`

Closes: #4726
Approved by: ankushagarwal

libra/libra,aptos-labs/aptos-core,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core