Add fixture path argument to `MarkdownFeature.new`

This default will be overridden in the EE-specific subclass that uses a
different fixture path.

spec/support/helpers/markdown_feature.rb

@@ -10,6 +10,12 @@
 class MarkdownFeature
   include FactoryBot::Syntax::Methods
 
+  attr_reader :fixture_path
+
+  def initialize(fixture_path = Rails.root.join('spec/fixtures/markdown.md.erb'))
+    @fixture_path = fixture_path
+  end
+
   def user
     @user ||= create(:user)
   end
@@ -122,7 +128,7 @@ class MarkdownFeature
   end
 
   def raw_markdown
-    markdown = File.read(Rails.root.join('spec/fixtures/markdown.md.erb'))
+    markdown = File.read(fixture_path)
     ERB.new(markdown).result(binding)
   end
 end