(#13261) Add key.pp smoke test

This commit adds a smoke test for key.pp, adding a key declaration
for the puppetlabs apt source.

tests/key.pp

@@ -0,0 +1,5 @@
+# Declare Apt key for apt.puppetlabs.com source
+apt::key { "puppetlabs":
+  key        => "4BD6EC30",
+  key_server => "pgp.mit.edu",
+}