$ZED_SYMBOL doesn't really work here once that will try to do something
like this:

  mix test MyModule.MyModuleTest

instead of using the path of the file:

  mix test test/my_module/my_module_test.exs
  
Release Notes:

- Fix mix test $ZED_SYMBOL to use ZED_RELATIVE_FILE instead
- Use ZED_RELATIVE_FILE instead of ZED_FILE to improve mix tasks results
on Elixir umbrella projects