Fix the assertion for not inclusion of turbolinks on application.js.

We were asserting if `//= turbolinks` was not present while the correct is
asserting that `//= require turbolinks` is not present.
Merge pull request #13003 from laurocaetano/fix_assertion_for_turbolinks

Fix the assertion for not inclusion of turbolinks on application.js.
Use a more generic assertion to make sure this tests will be useful.

Since we are using assert_no_match, if we change the code before
changing the tests, the tests will still pass and the assertion will
become useless