Add Rubocop ignore for Style/FileOpen cop

According to Rubocop the issue here is:
`File.open` without a block may leak a file descriptor; use the block form.

However, in this case it's a very specific set of tests that require
multiple files to be opened so the data can be loaded from static
fixture files.  The files are closed afterwards, so I've decided to
ignore the cop in this instance

Author: KPobeeNorris

spec/lss_loader_spec.rb

@@ -202,7 +202,7 @@ def create_a_single_office
     id
   end
 
-  # rubocop:disable Metrics/AbcSize, Metrics/MethodLength, Metrics/ParameterLists
+  # rubocop:disable Metrics/AbcSize, Metrics/MethodLength, Metrics/ParameterLists, Style/FileOpen
   def load_from_fixtures(members_csv_filename: "empty",
                          locations_csv_filename: "empty",
                          opening_hours_csv_filename: "empty",
@@ -236,7 +236,7 @@ def load_from_fixtures(members_csv_filename: "empty",
     volunteer_roles_csv&.close
     local_authorities_csv&.close
   end
-  # rubocop:enable Metrics/AbcSize, Metrics/MethodLength, Metrics/ParameterLists
+  # rubocop:enable Metrics/AbcSize, Metrics/MethodLength, Metrics/ParameterLists, Style/FileOpen
 
   def load_from_fixtures_with_error(**)
     expect do