(PUP-10922) Copy fqdn fact before mutating it

It's not possible to mutate the fqdn fact, because facts are frozen when added
to topscope[1], so make a copy when downcasing and update the test.

[1] https://github.com/puppetlabs/puppet/blob/7.6.1/lib/puppet/parser/scope.rb#L811

Author: joshcooper

lib/puppet/parser/functions/fqdn_rand.rb

@@ -24,7 +24,7 @@
     downcase = !!args.shift
 
     fqdn = self['::fqdn']
-    fqdn.downcase! if downcase
+    fqdn = fqdn.downcase if downcase
 
     # Puppet 5.4's fqdn_rand function produces a different value than earlier versions
     # for the same set of inputs.

spec/unit/parser/functions/fqdn_rand_spec.rb

@@ -80,7 +80,7 @@ def fqdn_rand(max, args = {})
     extra = args[:extra_identifier] || []
 
     scope = create_test_scope_for_node('localhost')
-    allow(scope).to receive(:[]).with("::fqdn").and_return(host)
+    scope.compiler.topscope['fqdn'] = host.freeze
 
     scope.function_fqdn_rand([max] + extra)
   end