test_erb.rb: escape

* test/erb/test_erb.rb (test_token_extension): escape % in
  dedented heredoc for editors which do not support this syntax
  yet.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53681 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/erb/test_erb.rb

@@ -523,9 +523,9 @@ EOS
     end
 
     src = <<~EOS
-      <% tag  = '<>' %>
-      <%= tag %>
-      <%== tag %>
+      <% tag  = '<>' \%>
+      <\%= tag \%>
+      <\%== tag \%>
     EOS
     ans = <<~EOS