(Maint) Ensure that 'hiera-puppet' is loaded correctly

The test for the hiera_* functions showed an interesting error. If the
hiera_hash or hiera_array tests were run in isolation they would
complain that Hiera was not defined and then fail. If the hiera_include
tests were run beforehand, then the tests for hash and array worked as
expected. The issues seems to have been that the require that brought in
hiera was being delayed until too late. By making sure that hiera-puppet
is loaded as soon as the function is defined, rather than called, we are
certain that hiera will be around when we need it.