Add test for ssh connection

This commit intends to mock SSH server that not depends on
OS which the test running on for testing purpose.

tests/requirements.txt

@@ -14,3 +14,4 @@ black==19.3b0 ; python_version >= '3.6'
 flake8
 flake8-docstrings
 jaraco.windows==3.9.2
+mock-ssh-server >= 0.5.0

tests/unit/remote/ssh/test_connection.py

@@ -0,0 +1,44 @@
+import mockssh
+import os
+from pytest import yield_fixture
+
+from dvc.remote.ssh.connection import SSHConnection
+
+here = os.path.abspath(os.path.dirname(__file__))
+
+user = "user"
+key_path = os.path.join(here, "{0}.key".format(user))
+
+
+@yield_fixture()
+def server():
+    users = {user: key_path}
+    with mockssh.Server(users) as s:
+        yield s
+
+
+def _client(server):
+    client = SSHConnection(
+        server.host, username=user, port=server.port, key_filename=key_path
+    )
+    return client
+
+
+def test_connection(server):
+    client = _client(server)
+    ls = client.execute("ls /")
+    assert ls
+
+
+def test_isdir(server):
+    client = _client(server)
+    path = here
+    isdir = client.isdir(path=path)
+    assert isdir is True
+
+
+def test_file_exists(server):
+    client = _client(server)
+    path = "/path/to/file"
+    file_path = client.file_exists(path=path)
+    assert file_path is False

tests/unit/remote/ssh/user.key

@@ -0,0 +1,27 @@
+-----BEGIN RSA PRIVATE KEY-----
+MIIEpgIBAAKCAQEA/HlMR0chgFLx6A/BmYi9Ypj9nr0kZ3Wo6n3ZdSXORJaH6e7d
+LSw+0eoD1NDCxDORZlwYNAg45zndU5sPN4IvVcAvC/FD0qSK1H0ku1p4QV82y4a1
+SOnzRlDUIVhdQxnlQa8gI+zDO8AyzOJ4oZ9LL7Hy+mIDqGxRwHeKfDYXuHkE+aM9
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+WB0qyFphvaazMlSIkn/3qv/lA9MQI5+e6MN3Cc8Qq9S3DE5GQzm1GycwGHeBTdZ/
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+J/s7F4duBZQdXWVkRrAX3WVVheqS30miE2OVp3nObmGbIQk5FRZi/HUO2BsHI6Km
+/c+AiJl3m90e91ZJ9nDmLJf9U+fYoCXgR4d/FcJtN2eV99ThmjumisvBMyIXVyy4
+zibVtC3i7cPes2P2nD83ZlHxAoGBAN+VSCckx4HXjAJH/ZSuvnriVdyaceD2ARF0
+jJxtCYzkoAAk3l6PaLMjUiw2exgcAkov2RbPkB/DKkqBSPHDliiAijWS3FpoHwFY
+XYaflj5yRHtdjYcwyWhaZvuLzvdeZppg7c3E14CMFn792IFSvTvW7AjWZBFbGdj8
+qpc+zY15AoGBANvsGFHmurP+VUu1ibegREBySts0WGNyq5VOTCrkN9S7AKR1pZNI
+I6W7KrRcXEvbBM7B47ykbE40hAachxN1Rpk+9qEom6etTaw/yMewgFNjZXYJw06z
+4bq6ofjKK8VqCWx41pWcmXj7Fa2A43RvZWg8TlX7Q8uc4wTBpTkuzfZC
+-----END RSA PRIVATE KEY-----

tests/unit/remote/ssh/user.key.pub

@@ -0,0 +1 @@
+ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQD8eUxHRyGAUvHoD8GZiL1imP2evSRndajqfdl1Jc5Elofp7t0tLD7R6gPU0MLEM5FmXBg0CDjnOd1Tmw83gi9VwC8L8UPSpIrUfSS7WnhBXzbLhrVI6fNGUNQhWF1DGeVBryAj7MM7wDLM4nihn0svsfL6YgOobFHAd4p8Nhe4eQT5oz0IKSJ0+pH9SrFqDTf7LnUwSmELTuavuasNviU6Z6rLxerRlxSMiSYqIYJVVEk7qDiRgXCvRTCkoLAvM5/DgXLZKnNZ97ch1VSoiuj3t3UAYL705RAROJWkFg5sEjWp9Ch5/oxP4IVxNXjRzIp0IbrHnt/qCk8xiph90dPJ user@unicorn