This commit streamlines all the initializers to simplify the
inheritance logic for:

- `Gitlab::SearchResults`
    -> `…::ProjectSearchResults`
    -> `…::GroupSearchResults`
    -> `…::SnippetSearchResults`

- `Gitlab::Elastic::SearchResults`
    -> `…::ProjectSearchResults`
    -> `…::GroupSearchResults`

The tests were also deduplicated and streamlined.