Add functional test for connection mode change verifying correct model information

[aaronbauman]

Expected behavior

• call \Pantheon\Terminus\Models\Environment::changeConnectionMode($mode)
• \Pantheon\Terminus\Models\Environment::get("connection_mode")
• expect ::get to return $mode

Actual behavior

• ::get returns previous mode, even when connection mode was successfully changed remotely

Steps to reproduce the behavior

• Implement a new terminus command in which connection mode is changed from sftp to git, and back to sftp (or vice versa).
• The "change back" function fails, because the Environment model property has not been updated

[aaronbauman]

Not sure if it's appropriate to set the property inside of ::changeConnectionMode, or use a callback to change it upon resolution of the sub-process.