Add tests for dotd command

[smarter]

It's currently not tested so we didn't notice when it was broken (see #3884).

[liufengyun]

We have a test in lampepfl/homebrew-brew :

lampepfl/homebrew-brew#11

That's where we found it broke. We could add some of the script tests to the Dotty CI.

[liufengyun]

There are also tests in packtest for each OS: https://github.com/lampepfl/packtest/blob/master/.travis.yml