Bump mix test.watch task

[lpil]

Tests run once on start, and we don't watch the _build directory any more.

We also watch Erlang files now too, but I suspect you don't really care about that one.

[reevoo-samuel]

_If you're new, read the guidelines._

Good code is tested, easy to understand and able to cope when things go wrong.

Good pull requests are descriptive, small and short-lived.

For the Author

• [x] Description explains what you have done and why.
• [ ] Tests have been written (or you have explained why you don't need tests).
• [ ] Documentation/comments have been added/updated.

For the Reviewer

• [x] Tests are green on CI (or pulled/tested on your machine).
• [ ] Tests cover happy and unhappy paths in the code.
• [x] The code is easy to understand.

[lpil]

You are fast, Sammy.

Liking the new message.

[jonnyarnold]

Ready to go :)

[lpil]

Continuing to meddle with Reevoo code. Woo