tests/helpers: Increase timeout and deal with newlines

[ffainelli]

TelnetControl can produce multiple newlines, make sure we filter through
those. While testing with a Broadcom BCM5301x it was also found that a
10s timeout was too short, bump it up to 20s.

[ffainelli]

The line splitting should probably be moved to Telnet control. I am seeing some weird problems with ser2net that need investigating. The timeout increase is good though.

[d-k-c]

Okay. Could you split that into two commits so I can integrate the timeout change?

About newlines, you made me realize it's a bit inconsistent at the moment. For SSHControl and LocalControl, It will return a simple string if the output is a one-liner, but a list otherwise. And for TelnetControl, I think it will always return a list. That's an issue that will bite us sooner or later.

[ffainelli]

Agreed, we should really solve this now so we have a consistent experience with all controls. TelnetControl still seems to suffer from some issues...