Mix integration fix

Updated Amrita mix task to be more inline with current Exunit mix task.

Fixes the below error and ``t_mix.rb`` tests:

```
2) test about `mix amrita` - supports running tests at a specific line number (Integration.Mix)
     test/integration/t_mix.exs:36
     ** (File.Error) could not write to file tmp/test/t_pants.exs: no such file or directory
     stacktrace:
       (elixir) lib/file.ex:635: File.write!/3
       test/integration/t_mix.exs:37
```