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.