test(sequencer): add unit tests for src/accounts/state_ext (#871)

## Summary
Second set of state-ext unit tests. In total there are seven files which
need tests:
- `astria-sequencer/src/api_state_ext.rs`
- ~`astria-sequencer/src/state_ext.rs`~
- ~`astria-sequencer/src/accounts/state_ext.rs`~ (this PR)
- `astria-sequencer/src/asset/state_ext.rs`
- `astria-sequencer/src/authority/state_ext.rs`
- `astria-sequencer/src/bridge/state_ext.rs`
- `astria-sequencer/src/ibc/state_ext.rs`

This PR just tests the `src/accounts/state_ext.rs` file.

## Background
It is good to have unit tests to ensure that the database works as intended.

## Changes
Unit tests for the functionality in the file `src/accounts/state_ext.rs` were added.

## Testing
:)