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/multitenancy_spec.rb

@@ -1,6 +1,6 @@
 require "rails_helper"
 
-describe "Multitenancy" do
+describe "Multitenancy", :seed_tenants do
   before do
     create(:tenant, schema: "mars")
     create(:tenant, schema: "venus")