This replaces the use of fake_application_settings with
`::ApplicationSetting.build`_from_defaults. The reason is that
`fake_application_settings` doesn't have the custom accessors that
`ApplicationSetting` has, e.g. `#commit_email_hostname`, thus this
can lead to unexpected `nil` values which comes from the database
column instead of `.default_commit_email_hostname` returned by
`ApplicationSetting#commit_email_hostname`.

Using `::ApplicationSetting.build_from_defaults` should be safe as it
doesn't try to `INSERT` a DB record, in contrary to
`::ApplicationSetting.create_from_defaults` which we used to use, and
which created issues that the introduction of
`fake_application_settings` tried to resolve (575dced5

).

Signed-off-by: Rémy Coutable <remy@rymai.me>