test: mint and burn

[rflechtner]

re/ KILTProtocol/ticket#3671

Adds unit tests for burning and minting. Tests document EXPECTED behaviour, which currently differs from the actual behaviour in a lot of places.

Currently failing:

• mint_into::mint_first_coin

  • Fails due to flooring of amount_to_mint during conversion to float; may be fixed by proper rounding, or ignored by changing the assertion to an assert_eq_error_rate! with error 1.

• mint_into::mint_in_refunding_pool

  • Should give PoolNotLive error but yields NoPermission

• mint_into::mint_large_quantity

  • Fails because we overflow when converting large amounts of collateral back to a u128. Can be fixed with conversion logic similar to the conversion to fixed.

• mint_into::multiple_mints_vs_combined_mint

  • fails due to imprecision - not clear if this can be resolved

• burn_into::burn_in_refunding_pool

  • Should give PoolNotLive error but yields NoPermission

• burn_into::burn_large_quantity

  • Fails because we overflow when converting large amounts of collateral back to a u128. Can be fixed.

• burn_into::multiple_burns_vs_combined_burn

• just like multiple vs single mint fails due to imprecision - not clear if this can be resolved

• burn_into::multiple_mints_vs_combined_burn

  • Tries to burn more than was minted, yields FundsUnavailable. Must be fixed by improving rounding!

• burn_into::burn_with_frozen_balance

  • Fails because it tries to re-freeze the account after draining it

• burn_into::burn_to_other

  • Funds are burnt from the beneficiary instead of the caller, which is wrong

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)
  • Either PR or Ticket to update the Docs
  • Link the PR/Ticket here

[Ad96el]

LGTM!