(#13690) Update 2.7.x for spec_helper compatibility with external projects

This basically entails:

• Introducing a new class Puppet::Test::TestHelper that will contain basic setup / teardown methods, which external projects can use to make sure that puppet is initialized properly during testing.
• Remove or abstract the Puppet::Util::Settings#initialize_for_tests / clear_for_tests methods in P::U::S
• Modify puppet core spec_helper.rb to use the new API.