(#6771) Stop stubbing some feature tests.

Now that the spec_helper uses the Puppet feature system after each test, the
over-zealous stubbing in this spec test causes problems.

It is, though, ultimately unnecessary - we can just define a feature with the
behaviour we want to test, and avoid all the pretend code entirely.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>