(Maint) Make rspec tests work with Puppet 2.6.4

(Maint) Make rspec tests work with Puppet 2.6.4

The behavior without this patch applied is that the rspec tests do not
function with Puppet 2.6.4.  This patch back-ports some fixes introduced
in Puppet 2.7.x to the spec_helper to make the tests work as expected on
Puppet 2.6.4.

The specific patches back ported from Puppet are:

 * 2ae88067 (Puppet::Util::ExecutionStub.reset)
 * cf183534 (Switched spec tests to use a class rather than Array as the
             log destination.)