The shell script in interfacer/contrib/setup_go.sh expands the $GOPATH
variable to download a script to $GOPATH/bin. However, if $GOPATH is not
set, that expands to /bin. If the script is run with root privileges
(accidentally), this will download a "strange" executable to /bin, which
is supposed to be for system executables. If it is run without root
privileges, it gives an (unclear) error about permissions. This commit checks
if $GOPATH exists. If it $GOPATH does not exist, it exits with error code 1.
After moving to Tcell and implementing its screen diff updates, we can
no longer watch STDOUT to get frames during integration tests. Instead
we need to use Tcell's SimulationScreen and which has a GetContents()
method. This is actually also a much more robust way of being able to
see what Browsh actually outputs during testing.