adds missing tests for Digestor#nested_dependencies

[rthbound]

I realized the tests were missing when I made an intentionally breaking change to ActionView::Digestor#nested_dependencies:

-        dependencies.empty? ? dependency : { dependency => dependencies }
+        dependencies.any? ? dependency : { dependency => dependencies }

That change, while obviously breaking, did not result in any test breakage.
I've added the two test cases that should have broken... My intentional breakage causes the tests added here to fail as follows:

  1) Failure:
TemplateDigestorTest#test_getting_of_doubly_nested_dependencies [/home/rth/Software/rails/actionview/test/template/digestor_test.rb:140]:
--- expected
+++ actual
@@ -1 +1 @@
-[{"comments/comments"=>["comments/comment"]}, "messages/message"]
+["comments/comments", {"messages/message"=>[]}]



  2) Failure:
TemplateDigestorTest#test_getting_of_singly_nested_dependencies [/home/rth/Software/rails/actionview/test/template/digestor_test.rb:135]:
--- expected
+++ actual
@@ -1 +1 @@
-["messages/header", "messages/form", "messages/message", "events/event", "comments/comment"]
+[{"messages/header"=>[]}, {"messages/form"=>[]}, {"messages/message"=>[]}, {"events/event"=>[]}, {"comments/comment"=>[]}]

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @senny (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[senny]

@rthbound thanks 💛.