Don't double-load locale files

When we use `require`, Ruby makes sure that we only load the file once,
but since here we're explictly pushing things onto the load path we need
to do that check ourself.

Found using bin/rspec --warnings, which showed the following:

    /Users/dgmstuart/dev/swing-out-london/config/locales/en.rb:3: warning: method redefined; discarding old ordinal_day
    /Users/dgmstuart/dev/swing-out-london/config/locales/en.rb:3: warning: previous definition of ordinal_day was here
    /Users/dgmstuart/dev/swing-out-london/config/locales/en.rb:7: warning: method redefined; discarding old human_date
    /Users/dgmstuart/dev/swing-out-london/config/locales/en.rb:7: warning: previous definition of human_date was here

...since these methods are defined in the locale file which is being
loaded here.

spec/support/time_formats_helper.rb

@@ -6,9 +6,12 @@
 require "active_support/core_ext/time/conversions"
 require "active_support/core_ext/integer/inflections"
 
-# Allow reference to translations in isolated specs:
-%w[en.yml en.rb].each do |locale_file|
-  I18n.load_path.push(
-    File.expand_path(locale_file, File.expand_path("../../config/locales", __dir__))
-  )
+locale_files = %w[en.yml en.rb].map do |locale_file|
+  File.expand_path(locale_file, File.expand_path("../../config/locales", __dir__))
+end
+
+# Allow reference to translations in isolated specs,
+# but only load the files if they're not already in the load path
+locale_files.each do |file|
+  I18n.load_path.push(file) unless I18n.load_path.include?(file)
 end