DEV: correct parallel specs rake tasks

This used to work due to side effects.

`rake parallel:migrate` used to work very inconsistently and would only migrate
some of the databases.

This introduces the recommended change to db.yml so the correct database is
found based off TEST_ENV_NUMBER if for some reason we did not set it using
RAILS_DB

Also avoids a bunch of schema dumping which is not needed when migrating
parallel specs



DB number 1 is very odd cause for whatever reason parallel spec is not
setting it.

lib/tasks/db.rake

@@ -68,20 +68,23 @@ end
 
 # we need to run seed_fu every time we run rake db:migrate
 task 'db:migrate' => ['load_config', 'environment', 'set_locale'] do |_, args|
+
   ActiveRecord::Tasks::DatabaseTasks.migrate
 
-  Rake::Task['db:_dump'].invoke
+  if !Discourse.is_parallel_test?
+    Rake::Task['db:_dump'].invoke
+  end
 
   SeedFu.seed(DiscoursePluginRegistry.seed_paths)
 
-  unless Discourse.skip_post_deployment_migrations?
+  if !Discourse.skip_post_deployment_migrations?
     puts
     print "Optimizing site icons... "
     SiteIconManager.ensure_optimized!
     puts "Done"
   end
 
-  if MultisiteTestHelpers.load_multisite?
+  if !Discourse.is_parallel_test? && MultisiteTestHelpers.load_multisite?
     system("RAILS_DB=discourse_test_multisite rake db:migrate")
   end
 end