Fix frontend fixtures failing when metrics dir does not exist

What does this MR do and why?

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.

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