Missing Schema on custom field only during PHPUnit Test
You only get that error during testing because the core testing base classes add a config schema checker during testing.

You have 2 options, the first is the easiest, but is not recommended, and that is to disable the strict config checking in your test by setting this variable to false in your test class:

  /**
   * Set to TRUE to strict check all configuration saved.
   *
   * @see \Drupal\Core\Config\Development\ConfigSchemaChecker
   *
   * @var bool
   */
  protected $strictConfigSchema = FALSE;


This approach is not preferred because you then have invalid config schema data in your module. It would be far better to fix the schema.

Instead of city_state: in your schema file, something more specific like:

field.field_settings.city_state:


or since this appears to be display related, it might be

field.formatter.settings.city_state: