Don't leak global state outside of test example (pupilfirst#655)

It is possible, dependending on the order in which specs are executed,
that the Rails.application.secrets.multitenancy setting leaks out of the
context and affects other specs.

With around hook, global setting is restored after the test, even if the
test eventually fails.

spec/controllers/application_controller_spec.rb

@@ -10,8 +10,14 @@
     let!(:third_school_domain) { create :domain, :primary, fqdn: 'www.third.localhost', school: third_school }
 
     context 'when multitenancy is turned on' do
-      before do
-        Rails.application.secrets.multitenancy = true
+      around(:each) do |example|
+        begin
+          current = Rails.application.secrets.multitenancy
+          Rails.application.secrets.multitenancy = true
+          example.run
+        ensure
+          Rails.application.secrets.multitenancy = current
+        end
       end
 
       context 'when the current domain belongs to second school' do
@@ -40,8 +46,14 @@ def current_domain
     end
 
     context 'when multitenancy is turned off' do
-      before do
-        Rails.application.secrets.multitenancy = false
+      around(:each) do |example|
+        begin
+          current = Rails.application.secrets.multitenancy
+          Rails.application.secrets.multitenancy = false
+          example.run
+        ensure
+          Rails.application.secrets.multitenancy = current
+        end
       end
 
       it 'returns the first school' do