fix: update polkadot-js api to 7.2.1 (#809)

* update polkadot-js api to 7.2.1

* update PalletAssetsAssetBalance -> Account

* refactor the mock data for assets to keep things DRY

* lint and set proper return types for exported mock test-helpers

* fix: comment

* fix logic to avoid breaking changes

* fix pallet testing refactor

* refactor logic to avoid breaking changes

* cleanup code readability in asset-balances service

* Update return type and return value

* update inline docs

* update comment grumbles

* update inline docs for added asset types