Only seed tenants when necessary in tests

On my machine, seeding a tenant takes about one second, so skipping this
action when it isn't necessary makes tests creating tenants faster
(although creating a tenant still takes about 3-4 seconds on my
machine).

spec/system/admin/tenants_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-describe "Tenants", :admin do
+describe "Tenants", :admin, :seed_tenants do
   before { allow(Tenant).to receive(:default_host).and_return("localhost") }
 
   scenario "Create" do