```ruby
require "minitest/autorun"
require_relative "../lib/heel"

class TestShell < Minitest::Test
  def test_open_fail_in_non_mac_os
    exception = assert_raises ShellOpenError do
      Heel::Shell.open('')
    end
    assert_equal('Cannot open in non-mac system', exception.message)
  end
end```

Fix error class in test
user
```ruby
require "minitest/autorun"
require_relative "../lib/heel"

class TestShell < Minitest::Test
  def test_open_fail_in_non_mac_os
    exception = assert_raises Heel::ShellOpenError do
      Heel::Shell.open('')
    end
    assert_equal('Cannot open in non-mac system', exception.message)
  end
end```
assistant