Reduce number of requests in user segments tests

We were testing the creation of newsletters and admin notifications for
each existing segment, which IMHO is a bit overkill, considering how
slow system tests are.

So far we don't have any reasons to believe creating newsletters and
admin notifications will only work for some user segments, so we're
testing a random one instead.

Running these tests on my machine is now about 15 seconds faster.

spec/system/admin/admin_notifications_spec.rb

@@ -215,7 +215,7 @@ describe "Admin Notifications", :admin do
   end
 
   scenario "Select list of users to send notification" do
-    UserSegments.segments.each do |segment|
+    segment = UserSegments.segments.sample
     segment_recipient = UserSegments.segment_name(segment)
 
     visit new_admin_admin_notification_path
@@ -225,5 +225,4 @@ describe "Admin Notifications", :admin do
 
     expect(page).to have_content segment_recipient
   end
-  end
 end

spec/system/admin/emails/newsletters_spec.rb

@@ -162,7 +162,7 @@ describe "Admin newsletter emails", :admin do
 
   describe "Select list of users to send newsletter" do
     scenario "Custom user segments" do
-      UserSegments.segments.each do |segment|
+      segment = UserSegments.segments.sample
       segment_recipient = UserSegments.segment_name(segment)
 
       visit new_admin_newsletter_path
@@ -172,7 +172,6 @@ describe "Admin newsletter emails", :admin do
 
       expect(page).to have_content segment_recipient
     end
-    end
 
     scenario "Geozone segments" do
       create(:geozone, name: "Queens and Brooklyn")