Support --watch for "elm-test make"

[jfmengels]

elm-test supports a watch mode through elm-test --watch, which is extremely useful.

In projects where tests are slow and when I'm doing some refactoring (moving position of arguments, renaming, etc.), I would like to have the option to run elm-test make --watch to have a quick feedback to make sure I did all the necessary changes for my code and tests to compile.

Unfortunately, at the moment elm-test make --watch ignores the --watch flag.

Prior art: elm-test-rs supports this option.