refactor: api integration tests

[Daanvdplas]

Closes #123 & #120

• Remove helper functions to the bottom of the file
• improved return value when calling the contracts so we can assert on a success and error at the same time.
• Have a consistent assert schema: value, expected
• if a contract is reverted it is clear from which function it is coming from.

Open question that might improve things even more:
https://substrate.stackexchange.com/questions/11765/what-should-data-be-decoded-to-in-execreturnvalue

[evilrobot-01]

The link you included on SE is broken.

[peterwht]

Looks good, small comments to resolve.

GitHub is not letting me put a comment on it. But the if-chain on line 156 has a clippy warning.

Please resolve it.

match value.cmp(current_allowance) =>
    Equal => {...},
    Less => {...},
    Greater => {...},