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.

spec/requests/admin/flags_controller_spec.rb

@@ -37,7 +37,7 @@ RSpec.describe Admin::FlagsController do
 
   context '#agree' do
     it 'should raise a reasonable error if a flag was deferred and then someone else agreed' do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
 
       _post_action = PostAction.act(user, post_1, PostActionType.types[:spam], message: 'bad')
 
@@ -52,7 +52,7 @@ RSpec.describe Admin::FlagsController do
     end
 
     it 'should be able to agree and keep content' do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
 
       post_action = PostAction.act(user, post_1, PostActionType.types[:spam], message: 'bad')
 
@@ -69,7 +69,7 @@ RSpec.describe Admin::FlagsController do
 
     it 'should be able to hide spam' do
       SiteSetting.allow_user_locale = true
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
 
       post_action = PostAction.act(user, post_1, PostActionType.types[:spam], message: 'bad')
       admin.update!(locale: 'ja')
@@ -90,7 +90,7 @@ RSpec.describe Admin::FlagsController do
     end
 
     it 'should not delete category topic' do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
       category.update_column(:topic_id, first_post.topic_id)
 
       PostAction.act(user, first_post, PostActionType.types[:spam], message: 'bad')

spec/requests/admin/groups_controller_spec.rb

@@ -78,7 +78,7 @@ RSpec.describe Admin::GroupsController do
     let(:user2) { Fabricate(:user, trust_level: 4) }
 
     it "can assign users to a group by email or username" do
-      SiteSetting.queue_jobs = false
+      run_jobs_synchronously!
 
       put "/admin/groups/bulk.json", params: {
         group_id: group.id, users: [user.username.upcase, user2.email, 'doesnt_exist']