Use a more expressive name for time zone tests

"With different time zone" didn't clarify which time zone was different
nor what it was different to.

spec/models/milestone_spec.rb

@@ -44,7 +44,7 @@ describe Milestone do
   end
 
   describe ".published" do
-    it "uses the application's time zone date", :with_different_time_zone do
+    it "uses the application's time zone date", :application_zone_west_of_system_zone do
       published_in_local_time_zone = create(:milestone,
                                             publication_date: Date.today)
 

spec/models/poll/officer_spec.rb

@@ -127,7 +127,7 @@ describe Poll::Officer do
   describe "todays_booths" do
     let(:officer) { create(:poll_officer) }
 
-    it "returns booths for the application's time zone date", :with_different_time_zone do
+    it "returns booths for the application's time zone date", :application_zone_west_of_system_zone do
       assignment_with_local_time_zone = create(:poll_officer_assignment,
                                                date:    Date.today,
                                                officer: officer)

spec/spec_helper.rb

@@ -109,7 +109,7 @@ RSpec.configure do |config|
     travel_back
   end
 
-  config.before(:each, :with_different_time_zone) do
+  config.before(:each, :application_zone_west_of_system_zone) do
     application_zone = ActiveSupport::TimeZone.new("UTC")
     system_zone = ActiveSupport::TimeZone.new("Madrid")