Fix fungible and fungibles set_balance return value

[liamaharon]

There is a bug in the calculation of the return value of the fungible and fungibles set_balance default implementation.

The doc comment says it should return the new balance, but instead it returns requested_balance +/- balance_change.

I have tests in this PR that show the incorrect return value.

This test

cargo test --package pallet-balances -- tests::fungible_conformance_tests::set_balance_mint_success --nocapture

shows

initial_balance: 11
set_balance_amount: 16
expected_set_balance_return_value: 16
actual_set_balance_return_value: 21

and this test

cargo test --package pallet-balances -- tests::fungible_conformance_tests::set_balance_burn_success --nocapture

shows

initial_balance: 11
set_balance_amount: 6
expected_set_balance_return_value: 6
actual_set_balance_return_value: 1

Luckily (and I guess why this was not caught earlier), I couldn't find anywhere in Substrate or Polkadot that the return value for set_balance is used, so unless there is concern about ecosystem projects relying on the incorrect behavior I think this is safe to merge.

[Polkadot-Forum]

This pull request has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/april-updates-for-substrate-and-polkadot-devs/2764/1