tests: flaky ssh tests

[skshetry]

https://travis-ci.com/github/iterative/dvc/jobs/340412779

tests/unit/remote/ssh/test_connection.py::test_hardlink 
[gw1] [ 80%] FAILED tests/unit/remote/ssh/test_connection.py::test_hardlink 
tests/unit/remote/ssh/test_connection.py::test_copy 
[gw1] [ 80%] FAILED tests/unit/remote/ssh/test_connection.py::test_copy 

I thought this was only failing on my machine, so I was ignoring it. But, looks like it occasionally happens on the CI. On my machine though this always appears.

Also, an important thing to note here is that, locally on my machine. it only fails when I'm running all of the tests, but when I rerun failed tests, it passes.

[efiop]

Might consider making ssh_server fixture's scope extend to session. That might help, but I'm not 100% sure.

[efiop]

Nope, didn't solve https://travis-ci.com/github/iterative/dvc/jobs/340941135

[skshetry]

Now, it''s failing, because of course, mock-ssh-server uses select.select(), why not?

I'll try creating a fix upstream later when I am done with #3881.