(PUP-11752) Fix fqdn_rand_string_spec.rb test

Since Puppet `7.23.0` `fqdn_rand` uses the modern structured
`networking` fact instead of the legacy `fqdn` fact.

In this commit we mock the new fact if using `7.23.0` or later.

This is a rebase of
puppetlabs#1294

and incorporates Aria Li's original change and a variation on Josh
Cooper's suggestion for keeping compatibility with Puppet < `7.23.0`

Author: alexjfisher

spec/functions/fqdn_rand_string_spec.rb

@@ -57,7 +57,11 @@ def fqdn_rand_string(max, args = {})
 
     # workaround not being able to use let(:facts) because some tests need
     # multiple different hostnames in one context
-    allow(scope).to receive(:lookupvar).with('::fqdn', {}).and_return(host)
+    if Gem::Version.new(Puppet::PUPPETVERSION) < Gem::Version.new('7.23.0')
+      allow(scope).to receive(:lookupvar).with('::fqdn', {}).and_return(host)
+    else
+      allow(scope).to receive(:lookupvar).with('facts', {}).and_return({ 'networking' => { 'fqdn' => host } })
+    end
 
     function_args = [max]
     if args.key?(:charset) || !extra.empty?