Rely on Gitlab::Shell public API, not reading internal files it may use

It ensures we rely on `Gitlab::Shell`'s public API and not on its
internal. Since `Gitlab::Shell` is caching the value of its token, the
only way to get the correct token is to call
`Gitlab::Shell.secret_token`, not to read the
`Gitlab.config.gitlab_shell.secret_file` file!
Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/requests/api/internal_spec.rb

@@ -5,7 +5,7 @@ describe API::API, api: true  do
   let(:user) { create(:user) }
   let(:key) { create(:key, user: user) }
   let(:project) { create(:project) }
-  let(:secret_token) { File.read Gitlab.config.gitlab_shell.secret_file }
+  let(:secret_token) { Gitlab::Shell.secret_token }
 
   describe "GET /internal/check", no_db: true do
     it do