tests(agent,ssh): add integration tests to SSH connection

[henrybarreto]

In this PR, we are adding the initial integration testes between Agent and ShellHub instance.

• Connect to the server.
• Connect to server with custom identity
• Reconnect to it after losing its connection.
• Authentication
  • Password.
    • With custom identity
  • Public key.
• SSH connections
  • Pipeing shell and exec request on same connection
  • Interactive shell.
    • Connection
    • With Pty
    • Without Pty
  • Command execution.
    • Exit status.
    • Stdin, Stdout, Stderror
      • With custom identity
  • SFTP.
    • Lagacy mode (SCP).
      • From Agent to client.
        Error when download the file; seems to be some package internal.
      • From client to Agent.
    • From Agent to client.
    • From client to Agent.

Running it

To run tests, I use this command:

go test -v --failfast -shuffle on -timeout 1h ./tests -run "TestAgent"

Timeout is set to avoid the default one that is 600 second, --failfast to help to spare time and -shuffle to guarantee false positive due to a before test, and -parallel to define how many tests can run in the same time.