Load LintExampleGroup again in plugin tests

Plugins also use the spec helper from puppet-lint. Until
5f80b77 the expected directory
structure was spec/puppet-lint/plugins but that changed. This partially
reverts the change which means plugins don't have to move their files
around.

Fixes: 5f80b77

spec/spec_helper.rb

@@ -134,7 +134,7 @@ def subject
   config.include(
     RSpec::LintExampleGroup,
     type: :lint,
-    file_path: Regexp.compile(['spec', 'unit', 'puppet-lint', 'plugins'].join('[\\\/]')),
+    file_path: Regexp.new('spec[\\\/](unit[\\\/])?puppet-lint[\\\/]plugins'),
   )
 
   config.expect_with(:rspec) do |c|