The local variables list in /srv/foo/
contains values that may not be safe (*).

Do you want to apply it?  You can type
y  -- to apply the local variables list.
n  -- to ignore the local variables list.
!  -- to apply the local variables list, and permanently mark these
      values (*) as safe (in the future, they will be set automatically.)

  * testing : "YES"

この警告を抑止するためには次のような形式で safe-local-variable-values を設定しておく必要がある。

(add-to-list 'safe-local-variable-values '(testing . "YES"))

この挙動に関するドキュメントはこちら。 https://www.gnu.org/software/emacs/manual/html_node/emacs/Safe-File-Variables.html