Add new run_jobs_synchronously! helper for tests

Previously if you wanted to have jobs execute in test mode, you'd have
to do `SiteSetting.queue_jobs = false`, because the opposite of queue
is to execute.

I found this very confusing, so I created a test helper called
`run_jobs_synchronously!` which is much more clear about what it does.

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/advanced_user_narrative_spec.rb

@@ -22,7 +22,7 @@ RSpec.describe DiscourseNarrativeBot::AdvancedUserNarrative do
   let(:reset_trigger) { DiscourseNarrativeBot::TrackSelector.reset_trigger }
 
   before do
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
     SiteSetting.discourse_narrative_bot_enabled = true
   end
 

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/new_user_narrative_spec.rb

@@ -25,7 +25,7 @@ describe DiscourseNarrativeBot::NewUserNarrative do
   let(:reset_trigger) { DiscourseNarrativeBot::TrackSelector.reset_trigger }
 
   before do
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
     SiteSetting.discourse_narrative_bot_enabled = true
   end
 

plugins/discourse-narrative-bot/spec/discourse_narrative_bot/track_selector_spec.rb

@@ -37,7 +37,7 @@ describe DiscourseNarrativeBot::TrackSelector do
   end
 
   before do
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
   end
 
   describe '#select' do

plugins/discourse-narrative-bot/spec/user_spec.rb

@@ -13,7 +13,7 @@ describe User do
   end
 
   before do
-    SiteSetting.queue_jobs = false
+    run_jobs_synchronously!
     SiteSetting.discourse_narrative_bot_enabled = true
   end