fix require 'eww in testing

Author: Danny McClanahan

tests/markdown-test.el

@@ -3739,7 +3739,7 @@ Detail: https://github.com/jrblevin/markdown-mode/issues/79"
 
 (defmacro markdown-temp-eww (&rest body)
   `(progn
-     ,@(if (featurep 'eww) body
+     ,@(if (require 'eww nil t) body
          `((ad-enable-advice #'markdown-live-preview-window-eww
                              'around 'markdown-create-fake-eww)
            (ad-activate #'markdown-live-preview-window-eww)