[bundler/bundler] Add missing mock

I'm not sure this test is useful but since it's there, it should be
complete.

https://github.com/bundler/bundler/commit/3e350089db

spec/bundler/bundler/shared_helpers_spec.rb

@@ -263,6 +263,7 @@ RSpec.describe Bundler::SharedHelpers do
     end
 
     it "calls the appropriate set methods" do
+      expect(subject).to receive(:set_bundle_variables)
       expect(subject).to receive(:set_path)
       expect(subject).to receive(:set_rubyopt)
       expect(subject).to receive(:set_rubylib)