ember-test-selectors: utils/bind-data-test-attributes: Handle failing `set()` with assert instead of warning

Reference Commit After experiencing some issues after trying to add a data-test-selector to tagless components, I realized afterwards, by looking at the commit on master, that a warning was created to give some information. This is a great idea, but maybe not explicit enough (I didn’t see this warning, in the middle of other warnings). If you agree, I can make a PR to make an assert instead of a warn, to be more explicit and to force the user to fix it as soon as this is done. What do you think?