recommend ember s over ember test --server

this page: https://guides.emberjs.com/release/tutorial/part-1/automated-testing/
says to use `ember test --server` -- which is problematic for a multitude of reasons, and while it'd be better to fix those issues, most folks want one command to use.

`ember s` and visiting `/tests` is only half the resources of running both ember s + ember test, which is best for low-spec computers.