WikiPage#historical? not to depend on global repo

During testing, the `let_it_be` at the top of the WikiPage spec file
does not reset the repository between runs. That means tests depend on
global state. This change fixed this for one test not to inflate the
diff and to speed up review.

spec/models/wiki_page_spec.rb

@@ -773,8 +773,11 @@ RSpec.describe WikiPage do
   end
 
   describe '#historical?' do
-    include_context 'subject is persisted page'
+    let!(:container) { create(:project) }
+ 
+    subject { create_wiki_page }
 
+    let(:wiki) { subject.wiki }
     let(:old_version) { subject.versions.last.id }
     let(:old_page) { wiki.find_page(subject.title, old_version) }
     let(:latest_version) { subject.versions.first.id }