MySQLPrecreated db implementation which does not require admin rights

So one possible way to address this is to simply store db_seq in the ConfigRepository. However,  different threads may have different PRECREATED_DSN_PATTERN values, you'd need to store separate db_seq's for each pattern, and I can think of a few ways in which storing the valuing will have weird affects.

So my suggestion would be to scan the DBs to see which DSN#s are actually in use:

```php
if ($dsnPattern contains "{{db_seq}}") {
  do {
    $dsn = str_replace('{{db_seq}}', self::$db_seq++, $dsnPattern);
    $isNumTaken = false; // does another existing DB use this data-source?
    // FIXME: $this->instances should an injected reference to InstanceRepository
    foreach ($this->instances->findAll() as $instance) {
      /** @var Instance $instance */
      if ($instance->getDatasource()->toCiviDSN() === $dsn) {
          $isNumTaken = true;
          break;
      }
    }
  } while ($isNumtaken);
}
```

Of course... since your current goal is to support a single, hard-coded DB anyway, we could just comment-out the $db_seq stuff and focus on testing other aspects of the build?