Memoize `get_all_versions` to reduce number of DB queries in testssh-speed-up-get-all-versions-test

Every time we access `ApplicationSetting` (e.g. when creating a user), we
also run `ActiveRecord::Migrator.needs_migration`:

```ruby
def needs_migration?(connection = Base.connection)
  (migrations(migrations_paths).collect(&:version) - get_all_versions(connection)).size > 0
end
```

During tests, we memoized the `migrations` call, saving a lot of filesystem
loads. However, we left `get_all_versions` alone. This patch memoizes that
as well to reduce DB and memory loads for that table.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/24899#note_21207429

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