(#13261) Replace foo source with puppetlabs source

Prior to this commit, the source.pp test included an apt source
named foo which broke the apt-get update exec when applied on a
target system. This commit removes it in favor of the puppetlabs
apt source which is valid on all target platforms.

tests/source.pp

@@ -1,13 +1,12 @@
+# Declare the apt class to manage /etc/apt/sources.list and /etc/sources.list.d
 class { 'apt': }
-apt::source { 'foo':
-  location          => '',
-  release           => 'karmic',
-  repos             => 'main',
-  include_src       => true,
-  required_packages => false,
-  key               => false,
-  key_server        => 'keyserver.ubuntu.com',
-  pin               => '600',
+
+# Install the puppetlabs apt source
+apt::source { 'puppetlabs':
+  location   => 'http://apt.puppetlabs.com',
+  repos      => 'main',
+  key        => '4BD6EC30',
+  key_server => 'pgp.mit.edu',
 }
 
 # test two sources with the same key