Wrap apply_manifest into RSpec context; initialise litmus earlier

This fixes the following error by exposing the necessary PuppetLitmus context:

```
An error occurred while loading ./spec/acceptance/01_apt_class_spec.rb.
Failure/Error: apply_manifest(lsb_package)
NameError:
  undefined local variable or method `os' for main:Object
# ./vendor/bundle/ruby/2.5.0/gems/puppet_litmus-0.22.0/lib/puppet_litmus/puppet_helpers.rb:86:in `block in apply_manifest'
# ./vendor/bundle/ruby/2.5.0/gems/honeycomb-beeline-2.4.0/lib/honeycomb/client.rb:70:in `start_span'
# ./vendor/bundle/ruby/2.5.0/gems/puppet_litmus-0.22.0/lib/puppet_litmus/puppet_helpers.rb:41:in `apply_manifest'
# ./spec/spec_helper_acceptance_local.rb:17:in `<top (required)>'
# ./spec/spec_helper_acceptance.rb:4:in `require'
# ./spec/spec_helper_acceptance.rb:4:in `<top (required)>'
# ./spec/acceptance/01_apt_class_spec.rb:3:in `require'
# ./spec/acceptance/01_apt_class_spec.rb:3:in `<top (required)>'
```

spec/spec_helper_acceptance.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
 require 'puppet_litmus'
-require 'spec_helper_acceptance_local' if File.file?(File.join(File.dirname(__FILE__), 'spec_helper_acceptance_local.rb'))
-
 PuppetLitmus.configure!
+
+require 'spec_helper_acceptance_local' if File.file?(File.join(File.dirname(__FILE__), 'spec_helper_acceptance_local.rb'))

spec/spec_helper_acceptance_local.rb

@@ -5,16 +5,20 @@
 RETRY_WAIT            = 3
 ERROR_MATCHER         = %r{(no valid OpenPGP data found|keyserver timed out|keyserver receive failed)}.freeze
 
-# lsb-release is needed for facter 3 (puppet 6) to resolve os.distro facts. Not needed with facter
-# 4 (puppet 7).
-lsb_package = <<-MANIFEST
+RSpec.configure do |c|
+  c.before :suite do
+    # lsb-release is needed for facter 3 (puppet 6) to resolve os.distro facts. Not needed with facter
+    # 4 (puppet 7).
+    lsb_package = <<-MANIFEST
 package { 'lsb-release':
   ensure => installed,
 }
 MANIFEST
-
-include PuppetLitmus
-apply_manifest(lsb_package)
+    include PuppetLitmus
+    extend PuppetLitmus
+    apply_manifest(lsb_package)
+  end
+end
 
 # This method allows a block to be passed in and if an exception is raised
 # that matches the 'error_matcher' matcher, the block will wait a set number