Before this commit, the test was flaky: it relied on the Perl test process
sending the kill() system call before i3 exited. This can easily be triggered
by adding a sleep(1) after the “cmd 'exit'” line.
This is because with i3_autostart => 1 (the default), i3test.pm kills i3
or bails out if it can’t.
So, we instead set i3_autostart => 0 and launch i3 ourselves.
This will unfortunately still make the code kill i3 and bail out,
because launch_with_config updates the $i3_pid variable that i3test.pm
uses for tracking whether it should clean up i3.
The solution is to exit i3 by calling exit_gracefully,
which will make the i3test.pm state correct.
related to https://github.com/i3/i3/issues/3009
