Skipping `Proxy` tests via CLI.

We're seeing issues with our CLI tests for creating proxies, which
initially were being caused by the fact that we were trying to check the
length of a file handler object to see if there were any errors. I have
updated the method where this issue was happening as well as tried to
document it as much as I could.

Currently, the process of creating an 'ssh tunnel' locks `Paramiko` and
prevents the test from completing, so I have skipped all existing tests
accordingly.
Merge pull request #1299 from omaciel/fix-proxy-generator

Skipping `Proxy` tests via CLI.