formula: fix missing test resources.

Revert one change from https://github.com/Homebrew/brew/pull/17165.

It's unclear to me why this fixes the problem, but it does.

Fixes https://github.com/Homebrew/brew/issues/17203

Library/Homebrew/formula.rb

@@ -553,9 +553,7 @@ class Formula
   # ```ruby
   # resource("additional_files").stage { bin.install "my/extra/tool" }
   # ```
-  def resource(name)
-    active_spec.resource(name)
-  end
+  delegate resource: :active_spec
 
   # An old name for the formula.
   sig { returns(T.nilable(String)) }