add auto test closes #735

[nilsivanson]

Resolves #735

Changelog

Enhancements

• Auto test for mix test case runner will automatically run tests after a change is detected in the project files, which normally happens on save, which happens when losing focus on a changed editor tab by default.

[nilsivanson]

Not sure why this failed CI. Can it retry?

[KronicDeth]

Sometimes the tests that use JInterface run too fast it hits the restart interval. I probably should release a new version of intellij_elixir (the elixir package) to allow me to tune it with environment variables.

[KronicDeth]

Confirmed it works. I wonder if we should enhance it in a later PR to use --stale, so it only runs the tests that are needed to run.