(maint) Require file type needed to manage autosign settings

Previously, if the autosign spec was run in isolation, it would fail
because the spec stubbed Puppet::FileSystem.exists? before the file
type was loaded, resulting in:

unexpected invocation: Puppet::FileSystem.exist?('.../lib/puppet/type/file.rb')

This adds the require to the spec to ensure the file type is loaded
before we stub FileSystem methods.