fix: account migration

[orriin]

Fixes minor issues with the last account migration and vastly improves test coverage / sanity checks.

Issues fixed

1. I was decoding Account straight into

        pub struct AccountDataStruct<Balance> {
            pub free: Balance,
            pub reserved: Balance,
            pub misc_frozen: Balance,
            pub fee_frozen: Balance,
        }

instead of

        pub struct AccountStruct<Balance> {
            pub nonce: u32,
            pub consumers: u32,
            pub providers: u32,
            pub sufficients: u32,
            pub data: AccountDataStruct<Balance>,
        }

which caused free to be set to nonce, reserved to be set to consumers, etc.

2. I was updating storage using ::mutate while the state was still undecodable, leading to the new state being set in storage with everything except data reset to defaults (zero). Switching to ::insert and passing the entire new state resolves this issue.

Other changes

• Moved migration to dedicated file, now that it's getting large
• Greatly improved thoroughness of pre_upgrade and post_upgrade checks

[keithtensor]

Might wanna give this a more descriptive name.

[orriin]

I mean it is the only Migration in the account_data_migration module :P

[keithtensor]

Was this something you wrote? I was thinking how it was even possible to get an old lifecycle runtime upgrade hook from upstream to execute in our chain, but if you wrote a brand new one, it does the job just fine.

[orriin]

Yeah I wrote this from scratch.