Replace Puppet.initialize_settings with Puppet::Test::TestHelper

[domcleal]

TestHelper offers a public API for (re)initialising Puppet reliably,
which is preferable to using initialize_settings. It also avoids
conflicts when multiple libraries call the same method.

[rjw1]

Thanks for the PR one of us will hopefully look at this soon.

[rjw1]

I think this is good to merge. 👍
In my testing it adds 20% to our runtime. I think this is acceptable cost given it's the right way to do it.

@DavidS do you have any opinions about this change?

[DavidS]

@rjw1 it does clean out all the caches, so the impact on runtime is not surprising. Given that @domcleal has found this, I do expect he's run into one of those nasty edge cases where one test had poisoned the well for another one, in which case there is no good way around it.

[domcleal]

Luckily it wasn't so much a test, but I was using puppet-syntax together with the kafo_parsers gem - both used initialize_settings, and Puppet threw an error because it had already been initialised by the other gem.

Though puppet-syntax has a guard to check if Puppet's already initialised, kafo_parsers didn't, so using the proper setup/cleanup mechanism in both gems seemed the best way to fix compatibility issues permanently.

[deanwilson]

The code is good and the explanation makes sense so I'm going to merge it. @rjw1 we should add a ticket to get a new release issued.