Add basic spec coverage for User#admin?

Strictly speaking this doesn't test that the current implementation
returns `true` if the user has the "editor-admin" role for *any*
organisation. However, that's already a bit of a bodge until we decide
whether we want to properly support non-organisation-specific roles, so
I think this is fine for now.

Author: floehopper

spec/models/user_spec.rb

@@ -211,6 +211,12 @@
     include_examples 'role_check', 'school-student'
   end
 
+  describe '#admin?' do
+    subject { user.admin? }
+
+    include_examples 'role_check', 'editor-admin'
+  end
+
   describe '.where' do
     subject(:user) { described_class.where(id: '00000000-0000-0000-0000-000000000000').first }