mix test failed on macOS

[wingyplus]

Found 2 tests fail on macOS:

$ mix test
$ mix test
Excluding tags: [ci: true]

..............................

  1) test GenServer can use pty (OsCmdTest)
     test/os_cmd_test.exs:31
     ** (EXIT from #PID<0.416.0>) %OsCmd.Error{exit_status: nil, message: "echo 1 failed: unexpected port exit", output: nil}

.......

  2) test GenServer returns error on invalid directory (OsCmdTest)
     test/os_cmd_test.exs:19
     Assertion with =~ failed
     code:  assert error.message =~ "no such file or directory"
     left:  "dir failed: exec: \"dir\": executable file not found in $PATH"
     right: "no such file or directory"
     stacktrace:
       test/os_cmd_test.exs:22: (test)

........................

Finished in 1.2 seconds
64 tests, 2 failures, 1 excluded

Randomized with seed 875537

The macOS doesn't have dir command. I think we can replace with ls command to check this case. What do you think?

UPDATE: I open a new issue #8 for test GenServer can use pty case.

[sasa1977]

Yeah, not sure why I used dir instead of ls here. Do you mind submitting a PR?

[wingyplus]

@sasa1977 PR opened here #9. :)