Fix tests of special devices.

Regular files were created, as the 'mode' argument of os.mknod was missing. However, creating devices requires root; so we can't reasonably do that in tests.

Instead, we're using /dev/null instead of creating one. And while we're at it, let's also use /dev/zero (which, if not handled properly, will result in an infinite read).

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Migrated from D2706 (view on Phabricator)