Fix frontend fixtures failing when metrics dir does not exist

If `tmp/prometheus_multiproc_dir` did not exist,
`spec/frontend/fixtures/application_settings.rb` would attempt to
render a warning message with `prometheus_multiproc_dir` local
variable. Since this fixture didn't pass along this local variable,
the generation would fail with `undefined local variable or method`.

On some new installations, `tmp/prometheus_multiproc_dir` may not
exist. Since the fixture generation doesn't need this, we can just stub
the existence check to be `true` and move on with life.

spec/frontend/fixtures/application_settings.rb

@@ -13,6 +13,7 @@ RSpec.describe Admin::ApplicationSettingsController, '(JavaScript fixtures)', ty
 
   before do
     stub_env('IN_MEMORY_APPLICATION_SETTINGS', 'false')
+    allow(Gitlab::Metrics).to receive(:metrics_folder_present?).and_return(true)
     sign_in(admin)
     enable_admin_mode!(admin)
   end