Test new functions

[roidelapluie]

It would be great if PDK was able to generate unit tests for functions:

$ pdk new test profile_prometheus::generate_receivers
pdk (INFO): Test type not specified, assuming unit.
pdk (INFO): Using Ruby 2.5.9
pdk (INFO): Using Puppet 6.24.0
[✔] Examining module contents
pdk (ERROR): PDK does not support generating unit tests for "puppet_functions" objects.