Yes, I've used exercism to broaden my knowledge and I've recommended the site to friends and colleagues who wanted to learn programming.

You mentioned tests: on exercism you don't write unit tests by yourself, the tests are only for you to check your solution. It certainly helps to see how tests can be written in any language, but you aren't forced into a strict TDD style.

The best feature on exercism for me is to be able to see the other solutions and learn from them.

In HR you can see other solutions as well

Yes, great option.

One thing I like to have to write tests is actually practicing writing tests :)