Implement Psiphon test

Test that Psiphon 3 [0] is working properly.

The test must:

• execute the Psiphon 3 client command
• check that the output text of the command match the expected output text
• check that is possible to make an HTTP request using the Psiphon3 SOCKS proxy

Open questions are:

• should the test install the client too?
• which could be the possible failures bootstrapping Psiphon 3 client?
• what debug information can we get from the software that would allow us to understand why and how it is being blocked ?

Psiphon 3 has a python client that can be run in linux. [1] It uses SSH dynamic port forwarding to create a local SOCKS proxy. It can use the system ssh command or the one provided in the Server code. By default the SOCKS proxy listen on 0.0.0.0:1080, but this can be changed in the psi_client.py

Add documentation/script about the installation process here?

[0] ​https://psiphon3.com/en/index.html [1] ​https://bitbucket.org/psiphon/psiphon-circumvention-system/src/ea8284ad204971669dc4af5efabaa4acfcb3afbe/pyclient/?at=default