Add tests for json (un)marshalling of accounts

[rhuairahrighairidh]

json (un)marshalling accounts seems to have slightly subtle behaviour that can trip up chain launches or exports. A test covering marshalling and unmarshalling various account types would help catch issues.

Particularly ValidatorVestingAccounts and accounts with multisig pubkeys (https://github.com/cosmos/cosmos-sdk/pull/6054/files)