More tests for eager loading of hasMany associations

[groue]

This PR adds nothing but new tests for eager loading of hasMany associations. It happened that a few days ago I wrote a request and was just not quite sure it would work. It was working (GRDB associations are really neat), but it wasn't explicitly tested yet!

The newly tested requests have the following shape:

struct Source {
    static let middle = /* hasOne or belongsTo or hasOneThrough to Middle */
}
struct Middle {
    static let destinations = /* hasMany or hasManyThrough to Destination */
}

// Fetch Source and its destinations through an eventual middle record,
// with an `including(all:)` wrapped in a `joining(optional:)`:
struct SourceInfo: Decodable, FetchableRecord {
    var source: Source
    var destinations: [Destination]
}
let infos: [SourceInfo] = try Source
    .joining(optional: Source.middle.including(all: Pivot.destinations))
    .asRequest(of: SourceInfo.self)
    .fetchAll(db)