Investigate the way Moonbeam tests fee calculation

[dmitrylavrenov]

As a follow up of #584

paritytech/substrate#13110

[MOZGIII]

More relevant links:

• https://github.com/PureStake/moonbeam/tree/db4790c5501d12e74495bd7d1268c2076bc3d27e/test
• https://github.com/PureStake/moonbeam/tree/db4790c5501d12e74495bd7d1268c2076bc3d27e/tests

[MOZGIII]

So, the way they check the fees is pretty comprehensive.

Firstly, they have this:

https://github.com/moonbeam-foundation/moonbeam/blob/d2e6d820de1384eae2ab52d8099116d3a1222184/test/helpers/block.ts#L84-L325

Then, they have tests like this:

https://github.com/moonbeam-foundation/moonbeam/blob/db4790c5501d12e74495bd7d1268c2076bc3d27e/tests/tests/test-balance/test-balance-transfer.ts#L72-L80

https://github.com/moonbeam-foundation/moonbeam/blob/db4790c5501d12e74495bd7d1268c2076bc3d27e/tests/tests/test-balance/test-balance-transfer.ts#L94-L152

We now have the E2E test setup that enables us to implement the same kind of test coverage, if we choose to.

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An interesting thing to note here is they too ended up comparing the account balance before and after to test the fee.