Warn when Test.only is being used, even if there are failing tests

`Test.only` causes a warning to be shown in the test output that not all of the tests are being run. This warning is only shown if all the test pass however. I think it would be better if this warning was shown even if there are failing tests.

I was working on a project where elm-test was reporting only one failing test. Because of this I didn't realize I had left a `Test.only` and was incorrectly assuming that the rest of the test suite were being run. I didn't notice this until weeks later and while fortunately I didn't break anything, I could easily imagine a scenario where I'd remove the `Test.only` weeks later and discover many failing tests and no idea which code change was the culprit.

Edit: It occurred to me that I would have realized not all the tests were being run if I had looked at number of tests that had passed and failed. In hindsight it was silly of me not to notice this but I think it would still be nice to make the test output more explicit about not all tests being run.