DEV: Isolate multisite specs (#13634)

Mixing multisite and standard specs can lead to issues (e.g. when using `fab!`)
Disabled the (upcoming https://github.com/discourse/rubocop-discourse/pull/11) rubocop rule for two files that have thoroughly tangled both types of specs.

spec/components/s3_inventory_multisite_spec.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+require "s3_helper"
+require "s3_inventory"
+require "file_store/s3_store"
+
+describe "S3Inventory", type: :multisite do
+  let(:client) { Aws::S3::Client.new(stub_responses: true) }
+  let(:helper) { S3Helper.new(SiteSetting.Upload.s3_upload_bucket.downcase, "", client: client) }
+  let(:inventory) { S3Inventory.new(helper, :upload) }
+  let(:csv_filename) { "#{Rails.root}/spec/fixtures/csv/s3_inventory.csv" }
+
+  it "can create per-site files" do
+    freeze_time
+    setup_s3
+    SiteSetting.enable_s3_inventory = true
+    inventory.stubs(:files).returns([{ key: "Key", filename: "#{csv_filename}.gz" }])
+    inventory.stubs(:cleanup!)
+
+    files = inventory.prepare_for_all_sites
+    db1 = files["default"].read
+    db2 = files["second"].read
+
+    expect(db1.lines.count).to eq(3)
+    expect(db2.lines.count).to eq(1)
+    files.values.each { |f| f.close; f.unlink }
+  end
+end