Make the open schools request spec dynamic

This test started failing because the number of open schools seems to
have changed from 81 to 80. Here, it's not the number that's important
but that the number matches the quantity in the test set.

spec/requests/get_open_schools_spec.rb

@@ -1,13 +1,21 @@
 require 'spec_helper'
 require 'json'
+require 'csv'
 
 RSpec.describe 'GET /open_schools' do
+  let(:open_school_count) do
+    CSV
+      .read(Dir.glob('tmp/*test-set*').pop, headers: true)
+      .map { |r| r['EstablishmentStatus (name)'] }
+      .count('Open')
+  end
+
   it 'returns a list of open schools' do
     get '/api/open_schools'
     expect(last_response.status).to eq(200)
 
     resp = JSON.parse(last_response.body)
-    expect(resp['data'].length).to eq(81)
+    expect(resp['data'].length).to eq(open_school_count)
 
     expect(resp).to be_valid_against_openapi_schema('SchoolList')
   end