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/lib/backup_restore/system_interface_multisite_spec.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+require_relative "shared_context_for_backup_restore"
+
+describe BackupRestore::SystemInterface, type: :multisite do
+  include_context "shared stuff"
+
+  subject { BackupRestore::SystemInterface.new(logger) }
+
+  describe "#flush_redis" do
+    it "removes only keys from the current site in a multisite" do
+      test_multisite_connection("default") do
+        Discourse.redis.set("foo", "default-foo")
+        Discourse.redis.set("bar", "default-bar")
+
+        expect(Discourse.redis.get("foo")).to eq("default-foo")
+        expect(Discourse.redis.get("bar")).to eq("default-bar")
+      end
+
+      test_multisite_connection("second") do
+        Discourse.redis.set("foo", "second-foo")
+        Discourse.redis.set("bar", "second-bar")
+
+        expect(Discourse.redis.get("foo")).to eq("second-foo")
+        expect(Discourse.redis.get("bar")).to eq("second-bar")
+
+        subject.flush_redis
+
+        expect(Discourse.redis.get("foo")).to be_nil
+        expect(Discourse.redis.get("bar")).to be_nil
+      end
+
+      test_multisite_connection("default") do
+        expect(Discourse.redis.get("foo")).to eq("default-foo")
+        expect(Discourse.redis.get("bar")).to eq("default-bar")
+      end
+    end
+  end
+end