Add unit tests for custom AccountId to and from string conversions

[ascjones]

Follow up to #1010.

This was copied from subxt, which in turn was copied from substrate. The methods to convert to and from SS58 encoded strings are assumed to be correct. We should add some unit tests to double check that and protect against regressions.

[ascjones]

Actually there is already a unit test which covers this: https://github.com/paritytech/cargo-contract/blob/master/crates/transcode/src/account_id.rs#L216